What if the outlet smells rotten?

The smell of rotten cabbage comes from three nearby outlets and, in addition, midges fly out of them. The apartment is on the 3rd floor, sockets in the main 50 cm wall between the two rooms of the same apartment. Sockets were installed 3 years ago and almost all of this time were not used. On the other side of the wall, sockets are also installed, but everything is okay with them. What can it be and what to do about it?