There may be an intermittent fault with your system. I know on the LS400 The airbag system works in conjunction with the two front seat belt pretensioners. On Mercedes cars these need to be replaced once activated it may be the same on Lexus cars too- however the restraint system usually only works once the sensors on the vehicle sense a force the same or greater than a predetermined force for instance in a head on collision and shouldn't be activated with normal day to day driving have this checked out by a specialist.
