they don't leave their shell but after dead a few days the rotting process, disconnects them from the shell and water current can bring them out or other snail/fish can pull at them or what not and they pull out,..
i had a few do this , briggs, and rams also,
What do they look like when out of the shell?
well kinda elongated triangle the coiled part decaying a bit and wetter than the foot is kinda gooy,.
the opercolum is just attached to the foot this is what usually is pulled to detach the rest,.
sorry for the grossness, but you asked,..