Question to experts regarding stains after painting work

Hello everyone,

I have a question that I, as a non-expert in crafts, cannot answer. It is about the following: about three weeks ago, I painted a wall. After being on vacation for the past two weeks, I noticed that dark spots have appeared on the wall. Honestly, I cannot explain this, since these spots were not visible after the painting and after the wall dried. Can anyone imagine what caused these spots? Would a new coat of paint help or is this a more serious problem?

Looks like it was exactly where the slot for the power cables is. Possibly it was filled with some putty that is not compatible with your paint. Who did the work and was it done recently?

It looks like it is exactly where the slot for the power cables is. Maybe it was filled with a putty that does not react well with your paint. Who did the work and was it done recently?

I moved into my apartment last year. I suspect the previous tenant's renovation work took place at the beginning of last year. However, such discolorations were not visible before. The stains must have appeared after drying. Could it be that this was caused by moisture? It also seems somewhat suspicious to me that the stains are located near the cable ducts.

that the stains run near the cable ducts.

Cable to the light switch, then to the socket, then to the left to the next outlet. Did you prime beforehand?

Line to the light switch, then to the socket, then to the left to the next box.
Did you prime beforehand?

I didn’t. Do you have to prime over an existing layer of paint? I had assumed until now that the primer has to be applied to fresh plaster. When drying, the wall definitely did not have any stains. It was only after several weeks that these streaks appeared. Is it possible that moisture penetrated through the cable ducts?

For now, I have repainted the wall. I will wait and see if there is any change.

Thanks already for the tips.