I had issues with Snow Leopard where it wouldn't actually go to sleep. The screen shut down, but the power led stayed on, and it would drain the battery as if in use. This could be something similar.
Yeah, as I started, ideas kept coming and the test kept expanding. It was a cool exercise though. I feel like doing this kind of thing, while a waste of time, is good practice. It's helpful to go through the motions of detailed testing and reporting on simple little tasks like this, because those skills translate directly to more complex tasks.
I had issues with Snow Leopard where it wouldn't actually go to sleep. The screen shut down, but the power led stayed on, and it would drain the battery as if in use. This could be something similar.