I worked in a Music store when I was younger. The employee restroom was about a hundred foot walk down a hallway from the sales floor. Inevitably, you would make the walk and find out someone was already in there. I rigged up a magnetic switch on the bathroom door, and ran it to a box mounted inconspicuously, where it could be seen from the sales floor. It had a single green LED that lit up when the bathroom door was closed. It was so simple and effective. It changed the employees vernacular for using the rest room. If someone was in the toilet they were said to be "green lighting".
I worked there for quite a while, and the green light was still functional long after I left.