Wet wall downstairs possible leak?

[Househelp101]

I have recently gutted my downstairs bathroom I have replastered and discovered the back wall is wet. Before starting the project the paint had started to bubble on the back of the wall. I'm thinking that it is leaking from my toilet upstairs which is right above the downstairs bathroom. Around the toilet upstairs it is bone dry no sign of leaks. Could the leak be from inside the wall where the waste pipe goes through and leaking downstairs into the wall? Outside there is a heavy about of moss around the external waste pipes.



Or could it be caused by being poorly isulated around the waste pipe?



Please see photos of outside pipes and inside wall



I'm need to find a solution asap because the next step to the room is tiling and that can't happen until the wall is dry.



Any help is welcomed