Sounds like it's just a triple, switched outlet,(3 switches and 3 outlets on one plate)
A long shot, but is the area behind the plate full of ants nest or other debris causing a leakage?
Quote from the OP
"Some time after that the owner and his wife came and sat in my pub restaurant" (my bolding)
Seems to indicate that you had at least an interest in it even if not the owner..
Note I don't know the other establishment and consider my reading ability ok.