Sly did that once, managed to get a corner up and went trucking along underneath the fleece from one side of the cage to the other. Scared the crap out of Pooper!
Out in his pen there's a folded up lengthwise fleece blanket, so it's nice and thick and squishy to lie on. Occasionally he'll pull that up and go under, puttering along the fence under the fleece just to be a funny boy.