I really cannot imagine what this is and how it happened. It is a very local stain. I found the painting on the wall with the stain and unhinged it and put it in a different position to save from further damage . The wall seemed dry. The stain looks like someone pored water on the painting. Very local. The only thing I can think might have done this is the cable from the outside light. But the wall was dry ... I will go next time it rains and see.
