The timing of the arrival of the odor might mean that factors like sunlight on the roof and/or changing humidity in the air are factors. Or some other change in the environment, such as increased heat or humidity from using appliances in the kitchen below the room (if that is the case). You might do better posting this on the Home Repair forum.

Your story kind of reminds me of the rental home we just moved from.

Mystery urine smell, worse in the evening, the unused upstairs closets were horrible, nothing visible if you poked your head into the attic (via one of those little squares in the closet), no pets, no children.

My mystery was solved one evening at dusk. Our master bedroom had sliding doors that we never used. In fact we very seldom even opened the blinds. I was laying in the floor stretching, and for some reason that evening I had opened the blinds. As I lay there I saw a bird fly off, and another, and another. That's weird I thought. So stood up to open the door and take a closer look.... oh my, those weren't birds. They were BATS! And there must have been a hundred of them flying out for their evening meal... FROM INSIDE MY HOUSE!! Big mess too! Of course I'm not saying that's your problem, but it might be worth having it checked out.

Bat guano is a sweetish smell, very distinct.

My bet is pet (especially cat) urine in the wood floor under your carpet and/or the carpet pad. The fact that it is around the door and is stronger when the air conditioner turns off are keys. Cat urine smell is intense and can last for years and years. Pull up the carpet and use a blacklight to find the urine; clean with enzyme cleaner. Carpet holds so much nasty stuff even if it looks clean. Think about ditching it completely for hardwood floors with throw rugs. Much more sanitary and updated!

All interesting ideas, thank you. I think my problem is that each possible cause for the smell is sort of shot down by one or more of the clues. And that is making me hesitant on taking action on any of the solutions.

I think the bat option can't be ruled out. I don't know the smell of guano, but this doesn't smell sweet. My guess in terms of a rodent was old pee in the wood somewhere in the attic from rodents who have moved on.

It could be pet pee in the padding and wood floor. Im not sure if a cat ever peed in there but I think id see the spot. I know cat pee very well and this isn't cat pee unless the smell changed. My dog peed in there when she was on meds once. It was clear, odd smelling pee, and a lot of pee. This smell is closer to that, but it happen years ago. Why start smelling now? Again it doesn't change with warm days, cold days, rainy days, etc. And why doesn't that area smell when I put my nose to it?

I was kind of wondering if anyone else had a similar experience with a baby room, maybe having to do with the baby herself that I'm missing. If this is anyone, that would be most helpful. We could compare notes.

One of the clues I left out is that the smell was not as bad during a week that the baby was out of town. I poked my head in there a few times around this time of day and it didn't smell. But i didnt pay attention enough to call this a solid clue, it was before the problem was a big problem. This clue could mean it has to do with her or the door (the door was closed for a week, no in and out). But if the odor had to do with her, it would smell in the morning when she has a 12 hour soaked diaper, not at 5pm when she hasn't been in there all day. Ugh!

I think we will end up replacing all the carpet and clean the wood. Something we should consider doing for her anyway, its just the priciest solution of any.

You say that you sense the smell mostly in the air. Is it possible that this particular area is just an area where other household odors accumulate? We have two such areas in our house, where odors will collect and linger- one is in a central area in the entryway, the other is in a bathroom that is in a back corner. One of those I can see having limited air circulation, but the first seems like odors would move through quickly, yet they don't and odors from other parts of the house will essentially leave those parts of the house and collect in these. I was just wondering if it was possible that the smell is coming from elsewhere (say a nearby bathroom or laundry room?) and is hanging there? What happens if you put a small fan in that area later in the day? Would the smell still be in that area with the fan on or off?

The other thing that comes to my mind is bed linens- particularly any down or feather product. I've had issues on occasion with these getting funny/smelly odors that come and go, and it's very hard to pinpoint the issue, even when smelling the comforter or pillow directly. It's only by removing the item from the area/house for a little while, that we were able to pinpoint exactly what it was. I've noticed this problem more when the humidity is higher and it was worse later in the day when temperatures in the house had increased, and like I said, it smells fine some of the time and then other times there is a weird funk.

I hope you can find the source. It sounds very annoying.

I may have a new theory on this (to anyone still interested). It's not concrete but none of the theories work with every clue, so feel free to chime in if you've had a similar experience. The odor is worst near her door. Her hooded bath towel hangs on the back of her door. It is wet at about 7pm each night, and its hung there to dry. We keep it clean, but don't clean it every day, and we have a few different ones. It doesnt make much sense as to why it would start to smell when dry, 22hrs later at 5pm (maybe the humidity and temperature theories?).

BUT, I went to grab it one night at about 6pm, and her towel wasn't there. I also noticed the room didn't smell. Clue! I'd also think it didn't smell in the summer because the AC vent is right there, blowing on it and drying it faster. Another thing that makes sense. Now I must add that when you smell the towel it doesn't smell like this odor, it actually smells nice like her bath soap.

My dog is older and spends all day in our bedroom, so it kinda smells like dog. My bath towel is hung in the master bath, in the bedroom. My towel smells like dog after one use. This got really annoying, but once I started keeping the bathroom door closed (due to an exploring toddler), my towel doesn't smell like dog!

None of this explains why the odor is pee- like, but maybe it's not pee at all, just similar enough to make me think that. I'm going to keep the towel somewhere else and see what happens.

Is the towel getting the wooden door wet? Is the door smelling? Wet wood isn't pleasant to smell? Just a thought.

Sounds very strange but if all else fails get some "Liquid Alive" and spray everything in the room . You can buy it online at Walmart or amazon. Works great for urine and other bad things.

I may have found your answer:

Detection of Black Mold

Most types of mold have a very recognizable smell, much like that
of urine. Due to such a distinctive smell, most molds are easy to
detect. But black mold is not very easy to detect and often keeps
growing behind walls and under ceilings without being disturbed, as
people are usually unaware of its presence.

If you have stained ceilings, walls, leaky faucets, or have unusual
allergic reactions, then your home may have a mold problem, and it
could turn out to be Toxic Black Mold.

Removal of Black Mold

If you have any black mold growing in your home, then you should
get it removed and the infested area cleaned as soon as possible. Black
mold spores can be very dangerous to health, and thus you should get the
black mold removed by professional mold removers and cleaners.

Black mold, and the negative health effects that it has, has lead
to a lot of laws being enacted to protect people who may have been
exposed to black mold. If you are a tenant, then it is the legal
responsibility of your landlord to ensure that your premises are free
from black mold. Similarly, if you have purchased a home that is
infected by black mold, and you have suffered illness due to it, then
you may be entitled to legal damages.

-----------

If it is black mold you need to get it taken care of Immediately. it can make you very sick. I was just watching the Willis family on TV and they have black mold and are having to move.

Your towel theory is not unlike the problem I described with bed linens. That sounds like a good idea to remove them from the area and see if it helps. It was so peculiar in that I could not smell that weird odor on the comforter, but when I closed the comforter away in another area for a few hours- no more smell. Someone later told me that it was okay to add bleach to white down comforters (and ours aren't the super expensive ones), and since I've started doing that on a regular basis, haven't experiences the weird smell.

We also have intermittent problems with our towels, and honestly, sometimes the smell is 'urine-like' even though I'm certain it is not urine. Some bacteria must be very hard to kill, especially in things like towels, so it keeps coming back. I'm fortunate enough to have a washing machine with sanitize and steam cycles, so I use those regularly on the towels and it keeps them smelling good from wash to wash. Prior to having this machine though, I would add a little tea tree oil to the towel load- it has a sort of peculiar odor, but you don't really smell it once the towels are dried, and it has anti microbial properties. And if you have the opportunity to hang those towels in the sun to dry, that would be a good thing to try too.

Hope you solve your mystery soon.

Was it a plant by any chance? We had a plant in our office that would give off a terrible stench around 7 pm each night. I haven't been able to find a picture of it online.

Coming here off a Google search where I too have a werid ”dirty diaper” smell in my bedroom near a door/hovering smell and only in one room. It is not an aminal or mold (ive had both of those) and it sort of smells like wet feather pillows but all my pillows smell fine…. Wahhh.

Its your window AC. open it and give it a good cleaning and watch the smell go away. nasty black gunk builds up its mold by the way, so use gloves and mask. the smell will go away immediatly.