Male Chick Located
A local falcon watcher with a vantage point from a nearby office building has made a positive sighting of our wandering chick. The young male was seen perched atop the parapet (raised edge of the ledge) at the farthest point possible from the nest box.
This relatively precarious position underscores our decision not to access the ledge in search of the "missing" chick. Any disturbance might have startled the young bird into jumping, resulting in an unfortunate conclusion. It is somewhat surprising to find the young male up on the edge at this age but there are some structures present (lights, fixtures etc.) that might have given him a leg up, so to speak.
As we suspected the young bird has simply taken advantage of the spacious ledge to explore its surroundings. The observer noted at least one of the parents attending to the young male. Given the fact that his sisters are so much larger then he - remaining on his own might offer some relief in competing for food. Should he choose to, he'll be able to make his way back to the nest box.