think what happens is corrosion inside master cylinder stops piston going to fully relaxed position when brakes off ,so putting slight preload pressure wise on brake system, this on mine was just below px when brake light switch activates ,so what happens is u walk away from fsc , every thing switched off after a drive unbeknown to you there is still slight px in brake system after a while possibly down to residual heat there is slight rise in px we might be talking 1 psi here , but its enough to operate switch which puts brake lights on which was a right sod because by then covers had gone on fsc and i was none the wiser! cure, rehone internal bore of master cylinder new seal kit problem gone.