The innate sense which allows one who possesses it to find the restroom in a public building (e.g., restaurant, DMV, other place of business, et cetera) or even private home without being given directions. Some people have it, some do not.
To test if you have a bathroom sense, simply get up from your table at a restaurant you've never previously relieved yourself at and head in the direction you think the restroom is. If you are able to semi-consistently locate the bathroom, pulled by some natural gut feeling, congratulations.
-Hey, I'm going to go use the bathroom.
-Okay, it's back to the le-
-It's okay, I got it.
(Bathroom sense engages)