The device usually enters sleep mode (with the lights going off) soon after the engine is shut off. However, there are instances when they might stay on for some time:
There is some buffered data that still needs to be transferred from the device to the Force platform. The device will keep sending that data up to 10 minutes after the ignition shuts off. After 10 minutes, the device will be put into sleep mode. (Don’t worry about losing the data; the device will resume transmission next time you start the vehicle.)
The device is in the process of a “firmware over the air” (FOTA) update. Depending up the update, this might keep the device awake for up to 30 minutes (though usually not for that long). The drain on your battery will be minimal and shouldn’t be a concern.