Yes, mystery snails can "fly the coop". They look for a dry place to lay eggs and need enough space between the water line and the tank top to do it. Falling to the floor often cracks their shells.

There's a better way to seal up openings. Go to the craft section of Walmart. You'll find the plastic craft canvas there, for needlework. A sheet of it costs about 1.00. You can cut it up to fit any openings and tape it in place.

I also used it to cover the filter outflow for my baby BN, because they will get into the filter. I put an elastic band around the filter to hold the canvas in place.
I ransacked my tank looking for my BN!