We had this situation last week. I never did find it. DH petted the smaller cat & had blood on his hand. I held the cat down and went over every inch of him and never found a wound.

We have 2 cats. The older one doesn't like the younger one and tends to bite him. Poor younger cat has had 3 abcessses from bites. We thought maybe the cat had a bite but coudn't find it.

Good luck - if you have more than one cat, maybe one is bitting another one.

Could it be mouse blood ? :)

Sometimes the rest of the evidence is destroyed.

I thought maybe it was something they killed but couldn't find anything so figured I'd find at least something to hint they'd been eating out.

Both cats wrestle and make like they are going to bite each other but I haven't seen anything to indicate they had bit each other. I'll be sure to try and check them out after I see them being rough with each other.

If it's a perfectly round, undisturbed blood drop, my suspicion would be a urinary problem. UTIs and crystals can cause bloody urine that is expressed in single drops. Watch your cats carefully for signs of frequent trips to the litterbox or squatting outside of the box. If you see any such behaviors, get the cat to the vet ASAP.

Burst abcess? But the only one drop thing would be rare. I had a cat that liked to "cat wrestle" with his buds and frequently developed abcesses from the tiniest injury. I noticed one developing, made a phone call to the vet for an appointment after work and went off for 8 hours. I came home to what looked like a quart of grape jelly and ketchup being spilled by the cat tree. No sign of the abcess on the cat except for the tiniest little claw puncture wound. I mean the cat looked absolutely fine! It was only by really holding him down and going through his fur that I even found the little red spot. It was under his ear, so I figured that his shaking his head had magnified the distance and volume of the abcess contents. It wasn't that big a swelling! But it sure seemed to be gallons by the amount of splatter that I found. Just like the ear mite medicine that gets shook all over (while growling) when I release them.

I guess what I'm saying is that maybe your guy had a small abcess that was on a less noticible spot (and one not in a location to be shaken all over the house) and it burst, leaving him to clean up most of the evidence.

if you find blood around the house and you can't find a puncture wound on the animal, take the animal to the vet for observation and some blood work should be ordered paying attention to platelet counts.

no cats in heat right?

Is the windowsill wide enough that they sit on it or do they just look out of it? That would help to determine if you found a drop of blood from their hind end (maybe from a UTI) or head.

Have you noticed any sneezing, nasal discharge, change in eating habits, etc?

Both cats have been fixed. Both seem to be using the litter as normal but I have considered switching to a litter that will change colour if it detects a UTI. Sneezing seems to come and go but never really troubling. I'll hear them sneeze 2 or 3 times in the evening or morning but then the rest of the day/week nothing. I have an air filter in my room which I turned off because it makes the apartment cold but I turned it back on. Dustin sneezed more often when there was renovations down stairs but they are finished now. The kitten had a wet sneeze when she first came home but hasn't had any since then.

The blood actually looks bigger than one drop, probably 2-3 but its just in one single spot. I have found some that did look like jam and had me confused thinking maybe it was bloody poop. The spot on the window sill is where my cats like to sit and there it was smeared and you could see the red. The others were darker but I assumed it was because it was dry.

I think its an abscess because I find blood so infrequently with none on their regular sleeping areas and their eating/eliminating habits haven't changed (though they can be very subtle in these areas). Usually any growling or meow noises during wrestling comes from Muffin the kitten but I've caught Dustin doing it a couple times this week and I think she was too rough and bit him. I broke it up and checked Dustin and coddled him while telling Muffin she should be more gentle. I'm not sure how to teach her not to bite. I've seen her try to bite his hind legs during wrestling but I assumed it was part of playing and they don't bite hard. Is there an effective way to stop her biting while not discouraging them from wrestling?

I will not discount a UTI. I might move the litter out where I can see it and monitor them and get that litter that does point out UTIs. Is it bad to empty the old litter and put in a different type? They are using clay based right now and I believe if you do change them over you should taper them onto the new litter but would mixing the UTI litter with the clay litter make the UTI stuff not work properly? Is there a better type of litter for cats that is not terribly expensive?