Synth Retention Bureau
My God, I... I remember. I remember it all. From before. Zimmer. The Commonwealth. The Institute. My God, all those runners I brought down...”— Harkness, after his memory is restored
The Synth Retention Bureau is a special branch of the Commonwealth Police tasked with capturing escaped androids (or "synths").
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.
The Synth Retention Bureau is mentioned only in Fallout 3.