3rd time Blue Mystery Snail escaped/fell

[Crystal90]

This is the 3rd time my blue mystery snail has fallen out of the tank. After the 2nd time (when I figured out what happened) I covered all openings. I don't know what happened this time. I've been really careful when I open the lid to feed so I can't figure out how he fell out this time. He ended up in the floor air vent so I'm assuming he bounced when he hit the floor. Unfortunately I didn't find him for 24 hours or so. He's moving a bit, not sure if he'll make it this time.

So, does anyone have ideas on how he got out? something weird that happened to you? What am I missing?

Thanks, Crystal