Atarian7

Yup, I've had rats die inside the wall—big ones!—and I've come to know that smell very well.  If you happen to have a dog, they can be helpful for sniffing it out; that's how I located the last one.

We had a similar dead mouse smell behind a wall in my game room.   I have traps to try to kill them before that happens, but it still happened. I searched in the drop-down ceiling and around the ducts in that room's ceiling (its in our basement/game room) but never found it so it had to be behind the wall.    There was nothing I could do. It took several weeks for the smell to completely go away.  My sympathies.  

Cats a good at sniffing it out too. Ours used to scratch at the metal duct vents and look at us like "Hey - this shit stinks. This - right here. You should fix this."