Mystery Smell...

So, I have this weird "something died" smell in my apartment. The trash has long since been taken out, the kitchen sink cleaned, all dishes washed, etc. I've now located the smell as possibly coming from the living room and maybe the heating/AC unit. I swear if a mouse or something died in there, I'm moving out.

(We had this problem in my apartment in NYC. The exterminators put something out for mice and one died in the wall. This one part of the hall stank for months.)