Volkswagen Golf GTI 2016 Owner's Manual page 258

NOTICE
Leaving the extender attached to the safety belt buckle when
the front seat is occupied and the safety belt is not being used
will signal to the airbag control unit that the front passenger
seat is occupied and that the safety belt is being used. The
electronic control unit for the airbag system will then receive
incorrect information that will
- cause the safety belt pretensioner to deploy
unnecessarily in collisions.
- cause the front passenger airbag to deploy later in
collisions in which the front airbag would otherwise be
triggered earlier to help protect an unrestrained front seat
passenger.
A pretensioner that has deployed cannot be repaired. The
entire safety belt must be replaced.