If you don’t see the door, the snail is gone—dead and likely eaten.
Turn a black turban snail over and you’ll see a circular, flat plate sealing the opening—in fact, you may get to watch the black snail close the door behind it as it pulls into its shell.
The closed door (also called an operculum) seals the opening to the shell, protecting the snail from drying out or from being eaten. Notice the concentric circles on the door? The door grows as the shell grows to accommodate the increasing size of the opening: the circles are the growth rings.
Even if the snail’s gone, the shell’s still of critical value in the tidepool: please leave them for other animals to use.