Casey is obsessed with the litterboxes and making sure everything is properly buried. He also will wipe his front paws on the sides of the litterbox after he's done burying his deposits. After he's done his business, he'll cover it, wipe his paws, sniff the buried stuff, rebury it, wipe his paws again, sniff again, and repeat a few times until it's properly buried. It doesn't matter if I've just washed out the litterboxes or they've just had the litter topped off.