I had a customer years ago that had two tanks. 1 a Koi tank, and the other an Octo.... The Koi kept disappearing with no signs. She finally set up a video camera just to see. low and behold, the Octopus would climb out of her tank, down the front, across the floor and go "fishing" in the koi tank, then carry dinner back across the floor to her tank for an expensive snack.
Craziest thing I ever saw.