The Stazione Zoologica in Naples, Italy also includes a public aquarium. (One of the features was an electric ray in a petting tank. You couldn't get away with that in the US.) We had a large octopus in one of the display tanks who disappeared one night. The catwalks we used to feed the display animals were simply a set of boards laid over the tops of the tanks.
When we went searching, we found sucker marks drying on the boards and followed them. The octopus had gone past the dogfish tank (dogfish love to eat octopus) past the moray eel tank (morays also find octopus tasty), past the sea anemone tank (pretty but inedible) and dropped into the crab display where he reposed on a pile of empty crab shells radiating pleasure and satisfaction.
Many people don't realize that an octopus can clearly show its emotion. It is relatively easy to tell when an octopus is happy, sick, scared, curious or even horny by the texture and color of its skin, which it can control almost instantaneously.
After a few similar incidents, we moved this guy to a large tank in the common area of the research facility where he became a pet.
For those of you who may still doubt the intelligence of an octopus, let me continue. Our new pet loved being fed by hand. He also liked to grab my arm to get lifted out of the water and taken for a walk. They can survive cheerfully in the open air for longer than you might think.
His favorite game was to watch the door to see who came into his area. Octopods have extraordinarily good vision. If a stranger entered, he would quietly ease himself up and slightly over the edge of the tank (it was open at the top) and wait for his opportunity. Then he would use his siphon to jet a stream of cold seawater 15 - 20 feet to douse the unwary intruder. Then he dropped back into his tank an display the strong colors and hornlike skin protruberances that were his equivalent of giggling.