Yes, management can order you to leave the building after you've clocked out. At least here. I can't find the language inthe master, but I remember seeing it, so it's probably in a supplement. You were not on break and had no right to remain in a break area. However they do have an obligation to let you see a steward in the event you claim that you have a grievance over being ordered off the clock early, and if they do not honor that request they may be in violation of the following contract language:
UPS NMA 02 - 08
UPS NMA 08 - 13
"ARTICLE 4. STEWARDS
The Employer recognizes the right of the Local Union to designate Job Stewards and alternates from the Employer?s seniority list. The authority of Job Stewards and alternates so designated by the Local Union shall be limited to, and shall not exceed, the following duties and activities:
(a) The INVESTIGATION and presentation of grievances with the Employer or the designated company representative in accordance with the provisions of the collective bargaining agreement:
...
Recognizing the importance of the role of the Union Steward in resolving problems or disputes between the Employer and its employees, the Employer reaffirms its commitment to the active involvement of union stewards in such processes in accordance with the terms of this Article.
The Job Steward or the designated alternate shall be permitted reasonable time to investigate, present and process grievances on the Company?s property without interruption of the Employer?s operation. Upon notification to his or her supervisor, a steward shall be afforded the right to leave his/her work area for a reasonable period of time to investigate, present and process grievances and to represent a fellow employee concerning grievances or discipline so long as such activity does not interrupt the Employer?s operations. The Employer will make a reasonable effort to insure that its operations are not interrupted by the steward?s engaging in such activity. The Employer shall not use interruption of its operation as a subterfuge for denying such right to the steward."
In other words, rather than running from your shift manager you should have told her that you wished to grieve her order that you punch out in violation of seniority or other contract provision and requested the presence of a shop steward to investigate your grievance. It would then be up to her to afford a shop steward the the opportunity to investigate your claim, and any unreasonable refusal to do so would be itself grievable. Where you remained while they fetched the steward would be pretty much up to management, though if it were standing outside in the rain that would also be grievable, IMHO.