Another commentor describes the key handshake. The car emits a high frequency wake up signal, the key receives the signal and emits a low frequency unlock code.

Another commentor describes the key handshake. The car emits a high frequency wake up signal, the key receives the signal and emits a low frequency unlock code. It is hard to restrict the time of the unlock code because it's low frequency limits the accuracy of your clock. Could you Honeypot the cars wake up signal? If the car detects a delayed broadcast of it's wake up signal it could trigger an alarm or at disable keyless entry. The high frequency signal will have lower tolerances for a timing check. Or car makers can provide Honeypot keys. If the key receives a wake up signal it can alert the owner and disable keyless entry. The owner would put the Honeypots where they don't want their keys to activate.