I've had a couple of these RF beacons for a while now and I made a little video showing some of the basic features of these beacons. Since I made the video there have been a few improvements to the design, but they basically retain the same features when used as a lost model finder.
The RF beacons are available here for $50 and are a great addition to any FPV aircraft and will massively improve your chances of finding a lost model. It can be set to come on after a set period of time, or it can use the microphone to detect when there is no noise and then turn on, that way if you fly with a Long Range UHF system you can be sure that it won't interfere with it. It can also be used as a data modem for sending GPS or other telemetry, but I've never tested this function.