It was a dark, tight space, which octopuses enjoy. He may have entered it simply because it seemed more comfortable than where he was.

Are they certain Inky isn't still in there?

Yes.

> The aquarium’s manager, Rob Yarrall, told Radio New Zealand that employees had searched the aquarium’s pipes after discovering Inky’s trail, to no avail.

Do octopi have a sense of smell? Maybe it/he/she could sense it in some way, or just noticed that when water flowed in the room, it all went down that drain. "I'll go where the water goes!"

In that vein, how often do octopi get out of the water (except when being brought out by humans)? And, are they aware of "water" vs "not-water"?

I already lmgtfy.com'ed all of these... just adding to the discussion.

Scents do not travel back up a drain pipe, and I doubt that sounds or other effects do, either.

And while a lost human might reason about following a river to civilization, I don't think there are similar drains in the ocean. If it made that connection, I am seriously impressed! While that's a well known tactic for explorers, many lost humans don't know it and don't figure it out!

My leading guess, though, is that he observed workers pouring other ocean things, probably including some tasty octopus food, down the drain.

> Scents do not travel back up a drain pipe, and I doubt that sounds or other effects do, either.

Properly engineered drains are, especially if they have traps. You must hang around different drains than I.

Good question. Not sure if there's an answer. This wouldn't be a story for us to read if he'd simply been recaptured.