You might have to just wait for the corpse to decompse. Several years ago in a duplex I was renting, there was an awful smell. I suspected a dead mouse, but couldn't find it. Finally, I went up in the attic, and followed my nose to where a mouse had fallen in BETWEEN the walls and died. There was no way to get to it, so I just had to endure it. The smell was gone in a couple weeks.