Key ring which alarm when other half far apart : Mom-Kid Cares
Attach the small one to your child and keep the other one on one. When your child runs off 5 meters range, it alarms. If you don't hava a child, attach it to your valuable things.
(?/ via redferret.net/ brando.com.hk: $9.90)
I think it's kind of rate to use "distance" for trigger.
