Sounds exactly like the occupancy mat, as you've figured out. On the E39 there is only an occupancy sensor in the passenger seat, not the drivers. Not sure if the same applies to the E6x but I can't see why they'd change. The point of the sensor was to determine whether or not the passender side airbags needed to be deployed in the event of an accident - no point wasting ££££s airbags for an empty seat. The bypass "modules", will simply fool the car into thinking that someone's always in that seat, which could potentially be expensive if you have a crash. On older cars like mine, a crash that deploys airbags is likely to have written the car off anyway so I wouldn't be worrying about it. No such mat was fitted to the drivers seat as the driver is always likely to be there in an airbag event, obviously :P