And I've had crabs do the disappearing act but pop up the next day but I only have three out of eight crabs up! I lifted up the cocohut ( it's under the moss pit which is against the corner so it's pretty hard to reach and it's dark.) and there was one crab sitting against the wall because, *GASP* another deep tunnel! I brushed sand into it and Spring crab walked out.
I've only got Big Dude, who has been de-stressing in the moss pit for a month time wise ( and he was too covered in sand

Hoping that they didn't all sync their molts up

But is the tunnel Fever a seasonal thing? It would make sense for crabs to have some sort of hibernation/Brumation in the cold months to avoid freezing, and their bodies getting confused with the lack of dropping temps.