This corner is also where our shower is. However, there are no water pipes in the walls with the problem. (Shower head and faucets are on an interior wall.) The shower tile appears to be in good shape. The tile goes about 6 feet up. Above that, they used plaster (not drywall). We've had handymen, plumbers and roofers look at this problem, and nobody can figure out what it is. The roof seems to be in good shape with no obvious problems in the attic.
We even went so far as to drill a few small holes in the plaster above the tile to look inside the walls. The wood and plaster on the interior of the wall seemed to be dry. However, the inside part of the exterior wall was covered with beaded water, like consensation.
We live in Southern California, so this is not an overly wet climate.
If anyone has a clue as to what this may be, we'd appreciate the input. It's driving us crazy not figuring it out.