Electronic fence mode alert system and method

The location of a transponder or micro-transponder (MT) device can be determined relative to a remote locator (RL) device. The RL transmits a multi-frame ping to the MT when initiated by a user. The MT receives the multi-frame ping, and transmits a multi-frame reply that can be hollowed out to conserve transmit power and reduce interference between devices. The RL calculates a distance between the RL and the MT using the time-of-flight (TOF) between the transmission of a ping and the receipt of a reply. The user designates one or more threshold ranges for the MT relative to the RL. The RL provides a user alert whenever the MT travels outside a threshold range. Alerts can be provided through a number of mechanisms including an audible alert, a visible alert, a vibrating alert, an email message, an SMS message, an instant message, a pager message, a telephone call, etc.