An electronic lock box system includes a wireless portable transponder that communicates with an electronic lock box using a low power radio link. The portable transponder includes: a wide area network radio to communicate to a central clearinghouse computer, a motion sensor to activate its wide area network radio, and a connector to communicate with a secure memory device. The electronic lock box sends a hail message that is intercepted by the portable transponder; the hail message includes identification information. The portable transponder responds with a message that includes a time sensitive encryption key; the lock box authenticates this response message using its own time sensitive encryption key. If the messages are authenticated, the lock box sends an access event record to the portable transponder, and this access event record is stored in the secure memory device. If a wide area network is available, the portable transponder sends the access event record to the central clearinghouse computer.