The kind of person who would do that once would probably be the kind of person who would do that many times, thinking it's a clever way to bypass the system. However, being the operation who marks every toilet they see as red 100% of the time is just as if not more suspicious as the person who other people mark red after the fact 100% of the time.

If we really want to take it up a notch, we have ways to do so. Deviancy, countermeasure, arms race all over again - but few such arms races are actually worth the hassle.