Cetainly you cannot OC in a vehicle because by our state law possession inside a vehicle (or on top of) is considered concealing.
However, you can conceal on private property with property owner's permission, yes? Therefore, a person contracted to provide security could conceal a firearm in their vehicle while that vehicle is on the private property in question, no? (Just don't run out to MCD's for a breakfast sandwich without legally storing it!)
I think this was the intent of the OP's question albeit this must be a pretty large drugstore to have that much property that vehicle patrols are required?!?!