Hey bud! pun intended!
First you were caught where? the guard shack, which is where? UPS property! As far as maturity, if you have a family, yes it was very immature not to be more thoughful or cognizant of what you were doing that has put your job in jeopardy. You could have insisted on union representaion, and should have, but there's little to nothing a steward/ba, or anyone could do. They have the right to take you to be tested, a refusal, is an admittance of guilt.
At best they will allow you to enroll in a rehab program, which means upon completion, you will be subject to random tests, if you fail one, that's it. Usually that is allowed when an employee approaches mngmnt before hand to seek help, it is their choice in your situation. No doubt you see this as a minor offense, and in part it is, put it shows negligance on your part. You should have known what was in your pants, as going through detectors is nothing new to you dude! It shows a lack of caring about your job, to be caught like that.
That said, I do wish the best for you, and hope a fair resolution for you, and that you put it behind you, and acknowledge your own mistake to prevent such a thing happening again. The blame is yours alone friend, the guard did his job, you should have known to make damn sure your pockets etc...were clean friend!
Let us know what happens, sometimes they will give someone another chance for redemption, but they will likely put you on a limb and call it a last chance, and anything else and bye bye, sometimes they don't. There's honestly not much the union can do, so blaming them on this is not the problem. They can negotiate, but there's precious little to go with, you are at their mercy.You have to convince mngmnt you want to get right, and get back.
Good Luck!
Cole