The Synth Retention Bureau is a special branch of the Commonwealth Police tasked with capturing escaped androids (or "synths").
Background
Androids are usually used as slaves for the Institute. Eventually, some androids learned enough to self-determine themselves and the world around them, so many have tried to escape the Institute. Since androids are state-of-the-art and are designed to resemble an actual human in as many ways as possible - even bleeding is simulated - they can be hard to track-down and capture, so the Synth Retention Bureau was created to specialize in capturing runaway androids. They have even made androids specifically to be Synth Retention Bureau operatives, as in the case of Harkness.
Known members
Appearances
The Synth Retention Bureau is mentioned only in Fallout 3.