We live in a 1930s semi. This photo is of a patch that i have just discovered in the second bedroom upstairs in the corner or the window bay. Bedroom over looking the garden. I have had a look outside and cant see anything obvious. The wall is an outside wall so not share with the bathroom. The rest of the wall looks dry. There is nothing of note above the patch and the windows seem fine. Any thoughts on what could be causing this and who we call in to investigate? Thanks in advance!