I'll put it simply and say that my African Dwarf Frog, Orik, has been disappearing at approximately the same times each day. There are quite a few different hiding places in my tank but when I look in them either Orik's not there or I can't see him.
You seem to be saying "This is his routine" and if that is the case the only problem I see is that you are probably moving things around in the tank trying to find him - which would stress out all the inhabitants. Apparently he is only gone for awhile and there the next day so I don't see anything to worry about.