There is a wire screwed to the body wood under the middle pickup, that goes to ground. I'm assuming this helps with 60 cycle hum? Or maybe its a voodoo wire?
It's extra onboard ground like the bridge ground. Called a chassis ground, it's meant to dissipate crap onboard the device in addition to the main ground out the jack, into the amp, into the earth.
The only time I've ever seen a wire screwed to the wood of the guitar it was with conductive paint. Maybe you're guitar has a cavity shielded with paint and then another coat on top?
My Jackson has those in both the neck and middle. It’s coated with conductive paint. The other end went to the back of the volume pot presumably to ground the cavities behind the pickups.
It's a safety wire, so that if you disassemble your guitar while on top of a tall ladder, you can't accidentally drop all of your electronic components...