Delayed Headlamp Control
WARNING: This page is about a different car, the 2002 Buick Regal and 2002 Buick Century. However, it is still accessible from the selected car via links, so may be relevant.
The BCM has a delayed headlamp control feature that will leave the headlamps ON for a 90 seconds after the ignition switch is turned OFF, when the automatic headlamp are on and the headlamp switches is in the OFF position.