Just because you know an entrance is behind a toilet doesn't mean that you as a character can see how it's set up and hidden. Example: I know (in RL) that Las Vegas has, inside the Commonwealth, a Prohibition-era speakeasy style private room called The Laundry Room. This is a hidden room. Even if you knew where it was, doesn't mean you'd see the way it is hidden. This goes for many speakeasies as well as bars that are behind soda machines.
That is where being able to perceive things come in. That way in passing, you'll notice something doesn't quite mesh with the surrounding area (like the soda machine isn't making nose as a refrigeration unit should).
So, I guess the solution, if you want to be able to easily see the door/entrance yourself, would be to work on those peepers and skill on spotting things.