This has been driving me CRAZY lately. The solution as suggested by Golden Shoes and Steven Blunk works as advertised. Fan-fucking-tastic!
Link: Proximity-card cloning
Turns out those HID, etc. RFID-based proximity cards aren’t quite as secure as your employer thinks they are. $30 will build you a cloner. Add a bit more, and you can read and clone cards from a couple of feet away.
Feel safe yet?