It may be a defective light switch but you would have to test the switch before condemning it as faulty. Check your owner's manual to see if the car has daytime running lights or "automatic lights on feature".
This is my daughter's Taurus. Questing her further, I think the security system is triggering the lights, although the horn does not sound. She says the headlights & tail lights come on after the car has been parked for a while, stay on for a minute then goe out. This happens enough to run her battery down over night.