Models crash, and at long range, particularly when there are trees around, crashed model planes can be easy to lose.
 Audio Alarm
For short distances, an audio alarm can give the user a simple way to determine direction to the model.
 GPS Dog Collar
In the US, a recent product emerged which contains a GPS reciever on a dog collar, and sends out its location periodically in the 150mhz band to a receiver with a screen and map. This is an updated consumer counterpart to scientific wildlife-tracking devices, and its 'black box' nature can speed recovery of a drone in a crash where everything else goes wrong. The product should work for several miles.
 Cellphone tracker
A cellphone with GPS tracking capability may be an effective means of locating a lost model in areas with coverage, at an unlimited range.
The Automatic Packet Reporting System is a very low-bandwidth, low-usercount mesh network operating in the 2 meter amateur band, which was designed for short GPS location messages like this to be sent once every few minutes (sending every few seconds will tend to overload the system). Repeaters are sparse, but will tend to be placed on tall hills, and large areas are covered if line of sight is likely. Some of them are networked iGates, which broadcast their data stream to the Internet, from whence further relay possibilities can be derived.
A satellite modem tends to be very expensive for constant telemetry, but for an emergency finder signal could prove invaluable.