Hmmm... I just went through something similar a few weeks ago. The thing about that type of odor/gas is that it will rise and linger depending on air currents in the room. You may not be smelling it near where the source is. I will tell you that with our shower, we have been squeegee-ing the walls, and rarely use cleansers in the shower, and I have lots of thick hair that can get stuck in the drain (but not clog it) and what I found was that after I had cleared a big hair clump, the actual drain trap was stinky. I had been smelling it for a week or two- it smelled like sewer gas; I poured 1/2 cup of Clorox into the drain trap, let it sit overnight, and now is 3 weeks and haven't had it again. I think my trap simply got funky. Another time a plumber didn't put the house trap on tightly and one corner of the basement was where the gas collected... 10ft from the source. So look around, not just where the odor is, cause that kind of odor is rarely sitting right at the source! And in the event you haven't been using that shower, make sure there is water in the trap (which prevents the sewer gas from rising out of the drain).

FOLLOWUP! In July 2012- I had sewer gas leaking into my bedroom, bathroom, and then up to the upstairs level of the house for five months. It was awful. I have an 18 inch crawlspace and a raised floor. Under the house is dirt and the pipes. I had FOUR plumbers and TWO handymen check it out. They found NOTHING! They were too big to fit around the pipes!!! We had put in a new toilet, and I had that pulled up and put down again, just in case. but that was fine. We checked all the traps, and now keep them full of water, so they are fine. The plumbers gave me estimates of $4500 for a sewer cleanout installation in front of the house, and $6800 to repipe the bad pipes, if they found any.... I did not have the money, and they really had no plan, just guesses. Last August 2012, I luckily found a neighbor friend who was able to crawl around under the house. He was a little guy, and knew how to do plumbing and other repairs!! He found leaking and large cracks in the main drain sewer pipe at the ground level, around the back of the pipe, hidden from general view and he used epoxy to repair it. This pipe was attached to other pipes, and if it needs to be replaced it will be a difficult job. That pipe was directly below the downstairs bathroom. That reduced about 70 % of the smell and seems to be holding tight with no more leaking there. But sewer gases were still leaking from somewhere else, but not as bad, or as often. it still kept coming back about every three days.They centered in my bedroom, and seemed to come out of the floor. Oddly, from the beginning, the gases only happened around 1am until 5am, then stopped all day long, so no one could smell them, unless they stayed all night. From the first repair, the gasses only occurred about every three days or so, and not as severe, but still stinky. My friend came back into town in November, and went back under the house, and found stress marks on the larger pipe that attached to the MAIN drain sewer pipe, and ran towards the front of the house. This pipe traveled directly under my BED, and the cracks were basically under my pillow area. (What LUCK!) He had to replace that whole pipe, and to get it inside we opened the ground vents and slid it through, and the old pipe out. After he replaced that and all the fittings, the gases were gone! THANK YOU GOD!!! It cost me $800 just to investigate all this, and get nothing for the repairing of it from that! Plumbers were just ready to get paid, with no idea what was actually wrong!!! My friend only charged me $550 plus parts for this! I suffered through it for five months, and hope the sewer gases didn't harm me in any way! I sure appreciate the clean smell now!!! By the way, it was an acrid stinky burning smell. It did NOT smell like METHANE, or anything described as general sewage smells. It was just hard to breathe it, and would cause eyes, throat and nose to burn. I had a runny nose right up to the day the last pipe was replaced, and it all stopped. GOOD LUCK TO ANYONE STILL HAVING THIS PROBLEM. I learned far more about plumbing than I ever thought I would need to know!

If the shower hasn't bee used in a while it's possible that the trap has dried out, allowing sewer gasses into the house. The trap can be primed by pouring a few cups of water down the drain. These odors can be continuous or intermittent, they can vary based upon wet/dry cycles, or high/low barometric pressure weather systems moving through your region, or gusty/calm winds, etc. In winter some vent stacks can ice over, restricting flow. Birds sometimes build nests inside the rooftop vent too. I've never seen a nest in a vent, but I have seen iced over vents. As to the fireplace smell, depending on the envelope of the house and how well the damper seals itself, you can get air "cold stack" reverse air flow down the chimney. This can bring "wet chimney" odors into the house. Or dead animal odors. Fireplace odors can come and go depending on air flow, they don't necessarily have to be 24/7. Remember that the anode rod is a sacrificial rod, it's there to prevent the tank itself from undergoing decay. If the rod has been completely eroded, the tank may start eating away at itself.

I have wasted thousands of customer dollars chasing odors reported by tenants not responsible for paying the bills. After 40 years I am convinced that some people are really receptive to power of suggestion. If anyone in the room smells or hears something,they will hear it as well. Part of what leads me to believe it came from servicing child care buildings. After recieving a report an odor was throughout a building,I asked several people what it smelled similar to. The answers within a single building ranged from dead animal,electrical fire,natural gas,dirty socks,carbon monoxide(hahaha anyone smelled co latly?),wet paint,,,,,,,,,you name it. Visit same building 3 times over a month and it only got worse according to staff. Put a dirty rag a in manoise jar,applied a couple drops of ammomia and sealed it up. When the corprate office recieved a call about my incompitance,plan B went into effect. Carry said rag through building exclaiming"I think I found the problem inside the air duct. YEP! That's it,problem solved. I'm not saying that's the case here but but I can tell you that spending 4 or 5 hundred dollars on the problem often makes it go away. BTW,I noticed your odor wasn't described.