Cars left in Parking area

[Henri Meli]

I own an office building with parking spaces reserved only for tenants and their customers during business hours. In the last month or so, as I drove by the property at night and during the week end, I noticed that some people who work there have been leaving their cars in the parking lot, at the same spot for days.

At my instructions, Property Management dropped a letter to all tenants reminding them that only business vehicles are allowed to stay overnight. Other vehicles must provided a business reason why, and give a date when they will be removed. 

The tenants responded to the PM that vehicles that look like personal vehicles are business vehicles. I have a feeling they are trying to pull my leg.

Is there a way to figure out if a vehicle is a business vehicle or a personal car? Why would someone leave their vehicles in the office parking lot? Is there any type of liability I'm exposing myself to, by accepting personal cars to just stay there overnight?