We call the master bathroom the Penguin Pen, because a certain tuxedo cat likes both to attend the humans using it and to hang out there after lights out, quietly warbling about his sadness. This is not to say he and his housemates don't like the other bathrooms, but their doors are usually closed and thus off-limits as performance spaces. They will use them when available, so we always "check for penguins" before closing bathroom doors.
Katherine used the center upstairs bathroom this morning before kitten breakfast, and she left it open because Whitey slipped in with her. He was guaranteed to follow her down to breakfast, saving her the effort of shooing him if she waited to close it.
Later, Billy hopped on my desk to get a drink out of my (okay, his) water mug, causing me to notice it was low. After he refreshed himself, I refilled it from the still open center bathroom. While in there, I discovered a fluffy penguin lounging on the bath mat, who departed gracefully. I then checked the tub, lest Whitey was hiding behind the shower curtain.
Katherine mentioned I should also check the under sink cabinet for Whitey. This makes sense in general as Whitey loves to hide in our self-closing French-hinged cabinets, but makes no sense in that bathroom because the door hinges are misaligned. The doors to be lifted while closing to get them slide over each other and actually close. Even a small human with thumbs couldn't do that from the inside.
Whitey was inside the cabinet, with the doors closed.
Beats me how he closed them.
Update: I tested both doors. The right door is misaligned. the left door will open and close fine if the right is left untouched. I must always be using the right door.