Worrying looking bulge in tacho facia

[dazza]

Went for a gentle little bimble today and I noticed a bulge developing in the tacho facia by the screw. My first thoughts were that something was arcing out and getting hot causing it to melt the plastic but when I got home and took the clocks apart I couldn't see anything that would suggest this was the case. The circuit board looked ok and where the screw fixing was there was nothing that looked liked it would cause this. Has anyone ever seen anything like this before ? As I rode home I could see it getting bigger but no blown fuses, everything works as it should and no burning smell. I'm baffled.