When is digging "technically" safe? (For Rescue Crab)

[Motörcrab]

I got word the other day that this guy did not make it through his last molt.

Now I'm having concerns about my third purple that is in with my 75G. They 3 were rescues we got early April of last year. They had their own tank until late July when I set up the 75. Right away they all went and successfully molted after being down around 50 days. For about a month after they devoured everything food and calcium. I believe all 3 of them molted a second time around August. Late November early December they all went down to molt again. Two of them resurfaced about a month ago. No sign of the third yet. Basing everything from their previous molts between 30 & 50 days the last one should have resurfaced by now. We are currently over 100 days, over double of their other molts They always seem to take shifts of being up so I rarely see all 3 at the same time. They usually stay hidden until 11pm and are usually hiding by 6am. Plus they are similar in size and are all in the same tapestry turbos with 1-1/8 openings.