It's great that you pay attention to your tank.
I have had snails lay on the bottom of the tank for what seemed like an eternity.
You can always use the "nose" test. If he hasn't move in a day or two smell him. No jolke, pick it up and smell it. If he really stinks, he's gone. Or you can use the "touch" method. pick it up out of the tank and touch the shoe or "door". If he pulls it tighter, he's still alive.