Routine, automated signals
An app that runs in the background and ‘pings’ your location automatically at regular intervals. Wouldn’t use too much GPS power as it would quickly turn on and then turn off, the problem would be making the data un-boring. Mapped? I think a simple list of GPS coordinantes + time might be nice to have as auto-diary type thing?
If you go missing, your account is public and your last known ‘ping’ can be located.