That "too far away" stuff is a problem in many areas of the game. I ran into that in several locations, including the trophy room. And I missed that tube myself, the first time.
I never bothered with the zoo key myself, but I believe you'll find a small hut/room/whatever in the zoo area that's locked, and the zoo key will open it.
Some doors don't have a visible keyhole, but clicking the right key on the door will open it. Wiz 8 won't get any awards for consistancy
Nightowl2