I've seen both of them sitting up there in the corner, and sometimes rock looks wet, pee maybe? That, I know they do all over the place. Could just look that way when humidity is high. It's a secluded spot, cuz the plant has large banana leaves that hang down and extend over the rock, which makes that area a favorite burrowing spot. Perhaps they like their privacy or knowing no one can sneak up on them while they're doing their business.
No idea, but i know i won't be changing or moving that area, as long as i keep finding gifts there.