What is this strange phenomenon with the switch? (see details)

For ventilation my studio has a fan built into the ceiling which ventilates through the attic vents.

Last night in the dark I went to the studio as I remembered I had left the fan on. I did not turn on any lights since I know the way blind.

I flipped the flat switch to off and a flash of light emanated from the switch in the wall. I turned it back on- nothing.
Turned it off and smaller flash. I did this repeatedly and had a flash about every third time that ranged from a tiny
barely perceptible flash to one that was brighter.

Is this common for switches? If not what is the problem? (guesses welcome)

I have no idea how long this has been happening but I will assume it has been happening since I had the fan professionally installed by an electrician 1.5 years ago.