A few details may make this a bit more clear:
- It was broad daylight -- around 4PM
- The woman was white
- We live in a predominantly white neighborhood
- Zach is half-Pakistani, but looks Hispanic to most observers
Joahnna called the police and complained about this, and in the ensuing discussion, it turned out the woman Zach asked for directions thought that he looked "suspicious" and called the police.
Joahnna and I are at a loss as to what to tell Zach if he wishes to avoid being periodically stopped and checked. It seems as if the bar for "suspicious" is suspiciously low.