I'm not sure how it is in the states but I know the Alberta CA is thus:
Under article 5 : Stewards
5.2 a) By arrangement, grievances shall be processed during the normal working hours of the steward. A steward shall receive his regular rate of pay when grievances or pending grievances are processed with the employer or employer property, or at any other place which is mutually agreed upon by both the union and the employer.
b) If the employer representative is unable to meet the steward during the steward's normal working hours, the steward shall be paid for all the time spent during the processing of the grievance with the employer on employer properly, or at any other place which is agreed upon by both the union and the employer.
Now, as I recall the company policy is that if you're not on the clock you're off the property? I didn't read anything in the contract relating to this so it's UPS policy and nothing to do with union contract? I'm not trying to be a troll, just trying to find the loophole to allow the steward to be on property while not on duty. Unfortunately I couldn't find it in the Canadian contract.
Also, unfortunately, in that first sentence it states that the grievances are to processed during the normal working hours of the steward. Also it states unless it's mutually agreed upon, so to me that means that unless UPS says, 'ok, file a grievance...umm...NOW' you can't do it until both groups agree on the time to do it if you expect to get paid.
The ONLY thing I could think you could possibly do is stand off the property and wait for a driver to notice you (off the clock as you don't want him to get into trouble) and ask him questions then.
Or you could play detective and just sit and watch and just take notes. They can't touch you for being off property and 'watching'.
Now, at least in Canada, the trespassing law essentially is whoever is in charge of the property has the right to issue pretty much any stupid rule they want about trespassing. If they say you can only be on the property while standing on one foot, well, that's the way it goes. Unfortunately that means any sup can issue you a trespass warning and then physically remove you themselves if needs be. If you fight back then you get charged with assault. Although I doubt any UPS sup would have the guts to attempt to restrain someone then usher them off property without cops around.
As for UPS calling 911, I think the cops would be more pissed at UPS since it wasn't a life or death emergency. Call your sup an idiot and use the land line to contact police for something as petty as trespassing. Sorry, it's a touchy subject for me.
I know I'm still a newb but debating laws (which essentially what the contract is) is something I love doing. Hopefully all the vets won't slam me too hard

Not to mention the Canadian contract is completely different and completely sucks compared to the American one.