How far must we go to investigate worker’s claim of targeted vandalism?

Q: An employee claims someone stuck a screwdriver in her tire on building property, and she suspects it was the act of co-worker(s) who don't ‘like’ her. She knows there is no proof and won’t accuse anyone directly, but this has happened three times. We told her that she needs to call the police if it happens again, as the building is not responsible. We also gave her permitted parking for 30 days at company expense. I doubt co-workers would vandalize her car. Is there any more I should do? – Gay, Texas