I read an account some years ago about an aquarium that had a lab they kept new fish in to quarantine them, or put fish in to heal them if they were sick, etc. and they had fish disappearing from the tanks.
They suspected one of their employees was stealing the fish to sell them to collectors with salt water fish tanks looking for exotic fish, so they installed cameras.
In the middle of the night, they saw an octopus push aside the top of its tank, slither over to an adjoining tank, pull the top off, remove the fish, put the top back on, then it returned to its own tank and pulled the top of its own tank back into place!
I don’t know about this instance, but that indicates to me there is a level of intelligence octopi have that they aren’t given credit for!
A similar story: an aquarium needed to refurbish its octopus tank, so they reluctantly moved the octopus to their big reef tank where they kept the sharks, worriedthattheoctopus might get eaten. A few days later, one of the sharks developed a wound on its side. Then another shark was also wounded. Concerned that the sharks might have started attacking each other, they put a camera in the tank. Turns out, the octopus that had been cowering in a crevice of the exhibit was coming out at night, snareing a shark, taking a few bites, then releasing it.
Octopuses are probably the most intelligent animals on Earth...............
An acqaintenance of mine had a friend who had that happen to him. He set up camera to find out why fish were missing in tank next to the octopus tank.