Okay, I've figured out the problem. I believe I've discovered a bug.
(hopefully this hasn't already been discovered by someone else)
I went through the walkthrough right up to the point where I was stuck (to ensure I hadn't missed anything coming before). Turns out I never used the tape on the elevator sensor. I just walked to the right and the game allowed me to exit the scene. This caused the game to be unbeatable as the sequence w\ the creature popping out of the toilet would never trigger.
I re-loaded the game the the elevator flooding scene and placed the tape over the elevator sensor this time, and what a difference (the bathroom actually looks flooded with water now too--which it hadn't before).