This article is from the Final-Fantasy and other Square Soft Games FAQ, by email@example.com (Nick Zitzmann) with numerous contributions by others.
Return to the lab floor where you met Red XIII. Near the south part of
the screen is a passage with two Potions in it that leads away to the
right. It looks as if you'd have to enter this passage from the upper
walkway, but in fact, you enter it from the ground floor. It leads up to
the rest of the Shinra building's floors.