You bring up some good questions. My understanding is that the owner of the property may be able to CC on his own property (or does this only apply to a residence and or vehicle). Can a property owner delegate that right to you because you are acting as his agent? Don't know.
I would defiantly OC for the time being, your employer may be putting you in a situation that could be a legal nightmare for you and for him. It might get complicated real quik..You can't shoot him for just steeling property. If you happened to get injured, would your employer foot the medical bills? WCF would probably question your claim if you got injured doing the duties of a security guard when they have you listed as a janator...not the same risk as a rent a cop.
Can you detain him: I think you, as acitizen,can arrest/detain a person if you suspect they are or about to commit a Felony.