What if we expand the idea. Instead of a tracker on everyone, what if we use the electromagnetic sensor idea? Expand it. Perhaps Tess would allow us to use the Watchtower as a place to set the sensor itself, and we could make it diffused. It wouldn't be able to tell where people were, but if their unique electromagnetic field is in town, then we could detect it...
...theoretically.
Alternatively we could go old school and do a ringer list. Everyone register their phones and when someone vanishes, everyone's phone goes off. they don't have to answer it, just hit as button, any button and the fact is acknowledged by the computer dialing. Within an hour, everyone who has a phone could be located, as long as they are willing to acknowledge the signal.