Yes, it's official. Black stuff growing on your wall is bad with a capital B-A-D. Figured it was time to cut it out and see what was in the back.
A month ago, definitely leaking:
Last week. Uh oh. That black stuff doesn't look promising.
Step 1: Go to the hardware store to buy an oscillating saw.
Result: All they have is a cheap one. I don't buy cheap tools.
Step 2: Text your neighbor and ask if he owns a reciprocating saw.
Result: He does, but since you asked for the wrong thing, that doesn't help. I also have a reciprocating saw
Step 3: Drive to the nearest bigger town (30 minutes away) and buy a good quality oscillating saw.
Step 4: Clear as much stuff out of the kitchen as you can so that it doesn't get moldy plaster dust on it, before realizing that you don't have anything to cover the rest.
Result: Turns out your sister is in town and could pick up dirt cheap bedsheets at the local big grocery/stuff store, you just have to drive 23 minutes to her house to pick them up when she gets home. Better than 30 minutes to town and then you get to visit your sister.
Step 5: Cover stuff. Cut out part of wall.
Result: Discover that it is soaked back there but that it sort of looks like it might just be from the leaking toilet (which you fixed) and the leaking sink (which you fixed).
Step 6: Wait. See if it dries out. I am really not ready for a full upstairs bathroom/downstairs kitchen renovation. I want to see if I can hold out for four more years when the house will be paid off, at which point I can pay cash for the renovation I want.