Oh dear... that smell has a familiar waft about it...lol.
Do you have cats by any chance ? Sounds like you have a dead mouse behind there somewhere. Even if you don't have cats, a mouse/mice could still have got in.
One of my cats (I have 2) brings me "presents" every so often, but the little burger lets them go!! So they scurry off somewhere and hide. If the other one is around, he'll catch them for me. Have you ever tried to catch a mouse ?... lol. HOWEVER, I have found 3 so far on different occasions that have died and gone off .... and the smell is exactly how you describe. Not nice.
Let us know when you find it.... and if you don't... the flies might get there first.... ugh!! Good Luck!!