We were trying to replace old switch plate covers at our new house when we came across one that is a type I've never seen before. It's a light switch cover that looks like an ordinary cover except for a small bulge above the switch that's maybe an inch across and half an inch tall. It's kind of an elongated pyramid shape. Removing the cover reveals two wires coming from the box (one from the top right, one from the bottom left) that appear to be soldered behind the lump. There appears to be a small piece of what looks like PCB board material, but doesn't show any features aside from the points where the two wires are attached.
Does anybody have any idea what this is? I don't want to just yank the wires out (they are attached to screw terminals in the box) without knowing what they are supposed to do.
The house was build in the early 20th century, so the plate could be really old.