I thought my mystery snail was dead last week. He was sitting in the same place for a day, then just bobbing on top for at least a full day with his door not quite closed. It had to be a full 3 days that he seemed to be dead. Then I took him out and tapped on the door and he didn't shut it the rest of the way, so I figured he was dead. I mean, it had been several days with no movement and the snail was normally very active.
I took him out and put him in a Ziploc bag with cold tap water, then put that in a bag from the LFS and set it by the door. The next afternoon, I took him to the LFS to see if they'd be nice and give me a new snail, but they said no. So I went to throw the bag away in the trash can in front of the LFS. For some reason I peeked in the bag, and right as I did he stuck out his tentacles and started crawling around. Now at this point he had been sealed shut in an airtight bag with just about 2" of straight tap water for 24 hours, and it was about 90F outside and I had the top down on my car and the bag had just been sitting in the sun. It was a 15-min trip to the LFS and now I still had to drive back.
Anyway, I got home and put him back in the aquarium and he's fine. I guess next time I'll just assume he's alive until I see his guts fall out of his shell and start to rot.