I can't imagine what is causing the odor. I keep 8 snakes in my reptile room and never have any odor whatsoever. I check their tubs every morning and every night - if any of them have any urine or poop I immediately clean their tub. No smells ever linger in the snakeroom.
I use Aspen in some tubs and kraft paper in others.
Maybe the smell is coming from somewhere else other than the snake cage.
Good luck with solving this mystery.
X2 if it smells like a dead mouse then it probably is a dead mouse. Start looking around the cage first, since that's where you think the odor is coming from. I didn't even think of this til I read L.West post. Good call.