While the NLRB has been imposing something of an all-or-nothing rule for restrictions on access to the workplace by off-duty employees, for purposes of protecting employees from discrimination if the purpose of their entry is to engage in union-related activities, as you note, "this is a defense contractor handling top secret work and nuclear materials gives them a legal obligation to control access to the facility, possibly getting them some leeway with these rulings."
It is understandable why your employer wants to secure its premises from all unauthorized entry, even if by an off-duty employee. It is understandable why the union wouldn't necessarily wan to take an expansive interpretation of recent rulings back to the NLRB with a case that could result in its being generally narrowed, or significantly narrowed for employers who have special safety and security concerns or who are dealing with classified information. If your employer is put in a position of having to choose between letting employees re-enter or maintaining proper security, I expect that it would take the path of telling people who forgot their property, "Sorry to hear that, you can get it when you come back for your next shift."
It sounds like the you, the union and the employer chose to resolve the matter as a deviation from standard disciplinary process, such that you didn't have to touch on the workplace access rule at all. It's not even clear that you raised the issue of off-duty access when contesting your termination.
I suspect that, at this point, the union considers itself to be done with your case. You can talk to your union rep but, were you to file a new grievance, I suspect that you would be on your own. As for the money, you've already settled the issue.

