This spring I took 6 just hatched briareus from Debbie (dbbga) and put them in my 19 gallon invertebrate tank, which was full of amphipods. I saw the babies for about a week, then I didn't see them anymore and assumed they had all died, which made me very sad.
After about three months, we noticed there was a decline in amphipods. A couple of weeks later, I thought my hermit crab had died....and so on. Then my husband was cleaning the aquarium, scraping off some algae, when out of the liverock popped a little red octopus - actually, a fair sized little 4-month old octopus, mantle over an inch! All that time, for four months, she had been hiding! The tank is in the kitchen, and we look at it many times a day, and with a flashlight at night. And we never saw her, all that time. (Unfortunately, she lived only a couple of weeks after we spotted her).
So you can see how well they can hide. On the other hand, they do become curious and you can lure them out with food. Many smaller octos are very cautious and become friendlier later.
Nancy