Not a city slicker perhaps Spacey? :) Key could be behind the counter, with the employee; could be electronically unlockable by employee.
(Guess I don’t want to make a person waste time asking an employee, esp if they come back to a couple people now in line.) “Come back with a warrant” is too good to miss an opportunity on now that I read it in one of the top comments here!
Ahh… 5-20% of the time! Most commonly seen on portable toilets, the most horrible brand name of which is “Honey Bucket” (portajohn, portapotty more commonly said).
That design is vastly more common than any other, is it? In the 5-20%, it’s not red versus white dots, but some kind of indicator period. May say “Occupied” and rotate to blank, for instance.
Probably most rare is indicators on individual stalls within a larger single gender restroom. (Indicators on single-occupancy bathrooms are the thing you’ll see if any.)