Russian Bathrooms

Those were just KGB interrogation rooms disguised as bathrooms.

Not being familiar with Russian culture, I can only guess that the light outside is turned on to show the bathroom is occupied. On the other hand, I have no idea why the locks were put on the outside. Hopefully, this is not meant for someone to lock you in.

When the lock is open, there is someone inside,
when the light is on, there is someone inside, but they also put this outside for safe electricity

Maybe the lock on the outside is for someone ELSE to decide whether you are finished or not. Some people need help making up their minds.

I lived in Tajikistan, a country that builds to Russian standards, for six months. The light switch was outside.

The bathroom had several other curios features. The first is that we had no sink in the bathroom. The first day we moved in I wanted to brush my teeth and only then did I realize the bathrooom had no sink. I had to spit in the bathtub when brushing my teeth for six months.

The other feature was that the washing machine was just outside the bathroom. We needed a special hose that we attached to the bathtub tap. The drainage hose from the washer had to be placed in the bathtub. You could not take a bath and use the washer at the same time.

These are just two of the idiosyncracies you can expect to see in former Soviet buildings.