Thx a lot Mbar,

I dived down to the stains for a very close look and it looks like that the something is coming trough the plaster. I also saw 2 spots were the plaster has holes which appear to me as if there are leaves under the plaster. I called the builder and he promised me to come out to inspect the plaster. (Sure the question is when)
Well we will see what happens.

Again thx a lot for your help.