What should you do if you lose a hermit crab? I have a friend whose family put very large shells in for the crab to try, and it disappeared. That was a long time ago, so it's obviously dead, but what should you do if you haven't seen it for a few days? Is shaking the shells inappropriate?
Okay, there are several things I want to touch on here.
1) I'm not sure what you mean when you say they put in very large shells and it "disappeared". Do they suspect the crab escaped when they were adding the shells? Or could they just no longer spot it in the terrarium?
2) Depending on how long "a long time ago" was, the crab may actually still be alive. Hermit crabs are ectotherms (cold-blooded) and, especially as they get older and larger, they can go for quite a while without much food. Gluttony, my largest crab, will sometimes burrow into the substrate and disappear for eight months or more!
3) Hermit crabs often disappear when they are molting (shedding their exoskeleton so that they can grow). This is their most vulnerable time, so they tend to burrow into the ground/substrate to hide until their new exoskeleton has hardened. Again, as your crabs get older and larger, they will likely disappear for longer and longer time periods during molting, however, the frequency of their molts should decrease.
4) If a shell is inhabited, you should be able to see the crab inside simply by flipping it over. (You'll see their big pincer bocking the entrance.) I would not recommend shaking shells. This will only cause undue stress to your crabs and it's not some how going to "shake them loose". If anything it will make them retreat as far as possible inside their shell.
5) If your crabs have burrowed into the substrate, do not disturb them or try to dig them up. Disturbing a hermit crab during a molt can be fatal. I know it can be hard waiting for them to resurface, but just keep maintaining the terrarium and eventually they should re-appear.
6) Give your terrarium a good sniff! If you notice any sort of fishy smell -- especially a dead/rotting fish smell -- you probably have a dead hermit crab on your hands. Start looking for a body and dispose of it ASAP. The general wisdom though is that if you do not smell "the smell", nobody has died and your missing crab(s) should eventually re-appear. :)