The vehicle reservation checkpoints are manned during the periods that vehicle reservations are required (generally 6am-3pm in 2023).
If you enter before or after the reservation period, no pass is needed. If you have a pass, it is unused if you don't enter during the reservation hours.
In your example, the next 2 days they are in the park, the pass is also unused because they are already staying at a lodge or campsite or chalet where they are already accounted for.