[dictum]

>I could imagine a system where police departments send a notification: "The person 50 feet in from of you is a potential person of interest in an investigation. Keep him in your line of sight for 10 minutes and win a free iPad!"

I'd be surprised if it doesn't go like "The person 50 feet in from of you is a potential person of interest in an investigation. Keep him in your line of sight for 10 minutes or you'll be arrested for not collaborating with our investigation."

[eitally]

This, but from the villian's perspective, is the key plot premise of the Agents of S.H.I.E.L.D tv show, actually.

[Avenger42]

In either of these cases - the person of interest wouldn't notice a crowd of people following him around to either win an iPad or avoid arrest?

[dragonwriter]

The notification doesn't have to be a broadcast, it can be sent sequentially to one-two people at a time with minimal overlap.