Delayed Headlamp Control
WARNING: This page is about a different car, the 2003 Buick Regal and 2003 Buick Century. However, it is still accessible from the selected car via links, so may be relevant.
The BCM has a delayed autolamp control feature that will leave the headlamps ON for a 90 seconds after the ignition switch is turned OFF, when the automatic autolamp are on and the autolamp switches is in the OFF position.