Since the security guards are engaged by a contractor (security agency), there exists no employer-employee relationship between you and the guards. Therefore, you can ask the agency to call them back and provide a replacement.
Under normal circumstances, no trade union will grant membership to such contract workers. The trade union office bearers seem to have allowed it due to an oversight. However, within the agency, they can form a trade union because the bargaining occurs between the guards (the employees) and the Agency (the employer). A prime example is the union of G4S Security Services.
Regards,
Madhu.T.K