Hypothetically, if I didn't want people to bring firearms onto the premises of a company that I own, and I find out that an employee has a firearm in their vehicle it's getting towed and searched by the police. If a firearm is found, the employee is then terminated.
Just a disclaimer: I have a firearm in my vehicle at all times. My employer doesn't have clear premises rule with regards to the workplace as a whole.
The interior of the vehicle is not yours. The use of the parking lot is based on whether, or not the employee is doing his work to your satisfaction, and whether, or not you're willing extend parking priviledges to that employee based on his otherwise satisfactory work. If you tow the car w/o the owner's knowledge, you are liable for the charges and damages for the action. The police will not enter the vehicle, nor will they assist you with the illegal tow. If you tow the car w/o the owner's permission, you are guilty of grand theft auto.