One of my hermit has buried himself, but he is still in his shell. When they are molting, don't they

leave their shell behind? This hermit crab has been buried for about two months. I had him in an isolation tank, but last night I could hear him chirping and scratching. I thought maybe he couldn't get out. I thought the sand had become too hard and crusty for him. I know you're not supposed to dig them up, but I was worried something was wrong! So, I gently dug him up and he was still in his shell! I put him in the big tank, right on top of the food dish, and he promptly sought out a hidden away spot and dug himself back under! Any hermie experts out there? Is this normal behavior? Is he molting? Should I dig him back up and put him in iso?