I have a 3 story rental unit with 4 apartments. The apartment on the middle floor reported that the closet wall was wet. When I checked it the wall had mold and a moist/wet feel to it. The ceiling and other walls seemed to be dry. This is an outside wall so I thought there was some type of leak behind the wall. I pulled the sheetrock to to find that there were no pipes. The outside of the house is cinder block and I discovered that the sheetock was screwed to furring strips, no insulation.
I checked the outside, no obvious cracks in the paint or holes in the wall. The nearby old window was a bit wet but there didn't seem to be any moisture on the wall under the window or to the side wall. I thought perhaps the water was somehow penetrating the block so I used drylock to seal the wall, 2 coats, added some foam insulation and sheetrocked with greenboard.
The tenant just called, 5 weeks later to say the closet was very wet and moldy. Apparently they do not use the closet very often. Above the closet are stairs heading up to the 3rd floor. There did not seem to be any moisture on the stairs when I checked.
So how do I find the source of this moisture? Especially when I don't live there myself so I can check it daily. Any advice or ideas are greatly appreciated.