An electronic lockbox system includes a wireless portable transponder that communicates with an electronic lockbox 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 lockbox 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 lockbox authenticates this response message using its own time sensitive encryption key. If an electronic key is used to access the lockbox, the improved system effectively disables the electronic key during a showing, so that the electronic key cannot be used to access another lockbox until the correct dwelling key has been placed back into the lockbox.