Here's a SAR system that you can deploy into your scene. You'll need two events: a startup event, that runs to set up the environment, and a tick event, that checks everything's positions and continued existence. The second task is most easily done by having a unit within a region with a "unit remains in area" trigger set on it- the duration of the unit's stay = the interval between ticks. A short interval is recommended.
Note: "== PersistLib here" means that you should go and copy code to replace that.
Startup:
Tick event: