It's probably a dead rodent (rat or mouse).  Call exterminator, they can figure out where it is, and get it out for you. 

Hate to be graphic about this, but I have a very keen sense of smell (one of my many er.."talents"; really it is a curse...cause I smell stuff other people cannot, like a dog,really.  And...they usually find out in time..that I am right on the money).  However, I live kinda far from you, otherwise I'd give you a free SMELL ANALYSIS.  Good luck, lemme know what turns up.  Remember..check the fridge!!  It could be some leftovers, that you Left Over a little too long.

"Ridin' down the highway late last night, should'a looked left and ye shoulda looked right.  Didn't see the stationwagon car, skunk got squashed, and there ye are.  Got your Dead Skunk in the middle of the Road, Dead Skunk in the middle of the Road, Dead Skunk in the middle of the Road...stinkin' to high heaven".   (Loudon Wainwright III..."Dead Skunk").

Gene