They're not magnetic, they are activated by weight. Except for the ones at the drive thru's. They are close position motion sensors. They are probably going under your frame. McDonalds is still like that around here, but most other drive thrus have integrated two sensors now for higher vehicles.
I've never heard that magnetic thing, although I often see people flashing their bright lights at a stop light thinking that will make them change
84 CJ7, 258, shackle reversal, 4.56 gears, lockrite , moser one piece axles, 37" SSR's