Thanks, all. I actually found him earlier hiding under the newspaper that I put down as substrate. It seems that he had pushed his large water bowl a considerable distance across the tank somehow in order to nose his way under there, despite the fact that I have replaced his cardboard hide with a more appropriately sized terracotta one. Haha, this guy is really strong and VERY determined to get under the newspaper. When i refilled the water bowl, I returned it to its original location in the back corner of the tank over the newspaper. Now he's been nosing around the area trying to dig under the water bowl again. Ugh.