I'm in Steve and Jay's camp on this. The only way I know of to see inside the walls is to cut holes, and you don't really know between which studs the critter is. I've seen someone use a dentist's mirror to look inside small holes, but you still end up with holes in your walls.
Not that I want to spoil your appetite, but I worked on a project last month where the lady of the house was into raising Guinea Pigs. This lady was not the best with self disapline. One of the pigs "expired" and was not taken care of in a timely manner. The pigs were kept in a back room that had a door that was kept closed, but that pig still got really "ripe". The smell was horrendous for about two weeks. After that, it began to abate. A week later, it was gone.