Your virus is on a machine in the network, therefore you're in the network. At that point, it is a matter of exploring the network, fingerprinting systems, scraping for exploits, and attempting intrusions. Or, waiting until an administrator does something silly like attempt to use their privileges on the machine to accomplish some task. I believe this was exactly how the Sony hack was conducted.

Edit: Also, at some point the employee will be reissued a new key fob for the "broken" one and at that point they will enter their credentials into the network again on that machine giving you access.

Edit 2: I guess a procedure that could prevent this is to require I.T. check the serial number of a fob that has been reported as "broken" thereby verifying there hasn't been a potential intrusion.