Vanishing crab

So we got 2 crabs for my son's birthday. Within about an hour of bringing them home, one of them completely vanished. The other guy seems to be adjusting well. Is it normal for them to bury themselves for a molt so soon after getting into a new tank? There is no smell, and I couldn't find any obvious evidence of someone digging around. I don't think it would have been possible for him to escape the tank. It's a 20g, 6-8 inches sub, 74-80f, 80-85% humidity.