Over on Hackaday.io we've come across a project by "Tom" who has created a small tracking device which is located using an RTL-SDR dongle and directional Yagi antenna. The tracking device itself is a simple fingernail sized low power UHF transmitter that transmits short pulses about every second or so in the 915 MHz ISM band. Tom writes that the range is about 400m (line of sight) and with a small button cell battery the device lasts a couple of days with its 180 uA current draw. Presumably longer operation could be achieved by significantly reducing the pulse rate of the circuit.

To receive the tracking device an RTL-SDR is combined with a high gain directional Yagi antenna, a three level 10 - 30 dB attenuator and an Android phone running the RF Analyzer app. The idea is to simply use the attenuator and directional Yagi antenna to determine the direction in which the signal is strongest. That direction with the strongest signal will indicate where the transmitter is. Tom's video below shows an example of the transmitter and RTL-SDR based tracking setup.