No idea this could happen!

[Kronk]

A week ago i bought a snail, he was cruising around for a couple days then I noticed it hadn't moved for a few days so i picked up the shell and took a look inside. I saw a dark green disk which i assumed was his decaying foot so i removed the shell and tossed it. Last night i saw something on the glass that i thought was a nudibranch or slug on closer inspection there is no doubt that it was the snail without his shell! He was grazing on algae on the glass. I dont know if his shell got trapped between rocks so he bailed out or what but i have a few Q's.

Is this common? How long can he live like this? If i put in another shell will he move into it?

I dont think i have any predators except for possibly a few un-identified worms.

Thanks,
Ken