Don,
I noticed that when I took mine off the first time so I put it back on & never removed it again. After the accident, I removed the bra from the trashed hood & it was a miracle ... the paint under it looked like it was fresh from the factory. I can't 'splain it but I'm guessing that after 1 or 2 removals & cleanings, the residue will no longer appear. Maybe it's some kind of manufacturing coating on the magnet that needs to go away over time???
Tom