I'd way rather be home petting the dog.
We used to call it choking the chicken. I guess the terminology changed.
Anyway, contractually, you have not reported to work until you clock in.
And you can leave anytime you want after clocking in without getting fired if, and only if, you have a good reason. This means communicating it to your sup. Family emergency, sick, etc.
If you just walk out, UPS considers that you just quit. And unless you can provide a legitimate reason at some point in the near future, the union will side with UPS.
Leaving before you clock in is not a contractual problem. You did not report for work yet. It is treated as a no call. But do you really want to risk your job on whether or not the state panel is in a good mood. If the contract is not perfectly clear, it is up to interpretation.
If your sup saw you at the building, and saw you leave, he could technically fire you for job abandonment. You showed up for work but did not clock in and left. Contractually, you did not report for work yet. But physically, you did. Again, do you feel good about putting your job on the line based on how the panel "feels" today?
Will you get your job back. I give it 99.99 to 0.01. But would you be that 0.01?