One of the greatest things about studying behavior is it's diversity. Sure, terrible water quality could motivate an octopus to leave it's aquarium, but that's as much a guess as anything. Who's to say those two octos didn't kamikaze because something in the same room as the aquarium looked neat enough to warrant exploration? Perhaps the sound of a pump drove them crazy. Maybe it wanted another bite of iguana tail. Who knows?
By and large, I've found that bimacs stay put. When I visted the NRCC, their bimac tank was a large tub with no lid. When Forsythe took me over to the now defunct CephSource warehouse, I saw dozens of bimacs in equally lidless aquaria. I've lost count of how many bimacs I've kept, but only one ever wound up on the floor, and he was well into senescence.
The best advice is to get to know your octopus. In the beginning it's probably wisest to take precautions, and then, if you deem it reasonable, remove security measures to make your life easier (anything that you screen with any octopus-proof mesh IS going to clog sooner or later)
Rocking on, Jimbo