More specifically, each phone can be identified with a unique number, as it pings the nearest cell tower.

When that phone "just so happens" to have approached a drop box, moved back in the direction it came from, and then does that repeatedly over a short period of time, never having done that before or since, then yes, you can say a person with that phone was doing something at the drop box, and doing it several times.

All the other numbers (not mules) would just walk on by, or make one drop and leave, never returning.