Synth Component

The synth component is a junk item in Fallout 4.

Characteristics
A small device of unknown function found only on synths of all three generations. It can sometimes be found on corpses of settlers, indicating that they are Synths, although it is not clear if they are agents of the Institute or an escaped synth who has possibly been freed and memory-wiped by the Railroad.

The actual location of the Synth Component on the bodies of synths is unknown, but as Roslyn Chambers mentions, there is no physical way to tell a synth from a human without killing them, so it can be assumed to be located within the brain or some other vital organ. It is also possible that the Synth Component allows the Institute to "program" synths with vocal commands, as well allowing the Railroad to memory-wipe freed synths.

Locations

 * Carried by Amelia Stockton and Sammy.
 * Dropped upon death by Art, Danse, Jules, K1-98, Magnolia, McDonough, Sturges and Timothy.
 * Dropped by DiMA upon death during The Way Life Should Be.
 * Random settlement settlers and brahmin may be synths and carry a synth component as well. They seem to join settlements as synths and can randomly turn hostile and attack a settlement later in the game, as well siding with any Institute forces that attack the settlement.