Canary Guards

I don't know who coined the term "canary guards", but you've all seen them. Those are the security guards and the cops who walk around in front of buildings and around the grounds of installations.

They are the "canary" guards because if something happens to them, that may be the first indication that some really heavy shit is going to happen. So in that regard, they are like the canaries in the coal mines back in the day.

My question is this: Why don't they wire those guys with some sort of datalink that would transmit their vital signs?