the.com/passerby

means Someone who happens to be walking past a place when something happens, and is then asked to remember exactly what they saw.

from A straightforward compound built from 'passer' (one who passes) plus 'by,' stitched together in English in the late medieval period. It simply describes the act in its own name: a person passing by. No hidden trick here — the word is as literal as a word gets, which makes it almost funny how unreliable the actual humans it names turn out to be.