there is a minikit behind a rock. have you got that one?

Just keep on going along the outer part, or AND every character that's locked up and
guarded there's a canister on top.

But if you're looking for the one I think you're looking for, there is a canister behind a
pillar that a character that is not on it.

If you can't find it with that information, get the minikit detector, and keep on going
along the edge until you see a white arrow pointing to where it is.