The argument, which has some merit, is that certain folks like school bus drivers, postal workers, sanitation workers, and so on, are out in the same public spaces on a regular basis, and would be more likely to notice sudden differences in the environment than others who aren't as familiar with those places.

Of course, moving from "something is different" to "something is suspicious" is a value judgment ... and how well that value judgment can be made by non-professsionals is a matter of debate. Your mileage may vary.