Mysterious Ceiling Damage
As much as you may want to avoid it, the best way to find the source of the water leak is to open up the ceiling and trace the path back to the source. Water can run a long way across ceilings, joists, roofs, etc. So if the non-destructive investigations didn't reveal anything, it's time to do some investigative demolition. You'll also want to leave it open until you see the leak happen again and then wait until the fix it tested before closing it back up. The only positive in this is that you really should remove and replace the moldy parts of the wall anyway, so this is positive motivation to fix it right.