"Seems [Mother] has intercepted a transmission of unknown origin."
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Synthetic sleeper agents are androids programmed to conceal their true identity and/or purpose from those around them. While some sleeper agents will merely operate as synthetics programmed with ulterior motives concealed from their associates (and possibly the android itself), the advancement of android technology has allowed some synthetics to effectively pose as human, concealing their very nature from the people they operate beside. However, as many vessels, settlements and colonies are typically equipped with at least one synthetic, it is often not necessary to hide a sleeper agent's artificial nature.
The Weyland-Yutani Corporation was known for employing android sleeper agents, surreptitiously programmed to further the company's goals, often at the expense of human employees, who would be oblivious to the synthetic's real aims.
While a synthetic's basic programming would normally not allow it to harm or endanger a human — in line with the Three Laws of Robotics — it is entirely possible (albeit highly illegal) to install specific software in a unit that would override normal behaviour, potentially leading the affected synthetic to work against any superiors and/or peers who may pose an obstacle to its objectives. An unstable synthetic might go so far as to forgo human life in cases where it sees humans as obstacles in the way of its grander mission set by the corporation.
Owing to their insidious nature, synthetic sleeper agents are not typically based on commercial models. It is likely that such agents, particularly those required to actively pose as human, are bespoke constructs manufactured especially for their mission, as seeing a duplicate model would immediately undo any deception.
Ash was a Hyperdyne Systems120-A/2 Synthetic placed aboard the USCSS Nostromo as a sleeper agent in 2122, following the discovery of an alien signal emanating from LV-426. Ash's orders, unbeknownst to the rest of the Nostromo's crew, were to assess whatever lifeform might be found on LV-426, retrieve a live specimen and return it to Weyland-Yutani, at the expense of the crew if need be. In this regard, Ash's status as a synthetic was concealed from his crew mates, who only discovered his true nature upon his destruction at their hands.
While he himself was human, Weyland-Yutani employee Michael Bishop possessed at least one synthetic double of himself. This double was used in the interrogation of Dwayne Hicks in the real Michael Bishop's absence, and was ultimately sacrificed to delay personnel from the USS Sephora while Michael Bishop himself made his escape. However, the data stored on its hard drive was subsequently accessed by the Bishop android from the Sephora and used against Weyland-Yutani.
Karl Bishop Weyland, the overall administrator of the Weyland-Yutani research facility on BG-386, claimed to be a direct descendant of Charles Bishop Weyland yet was revealed to be an android upon his destruction. Furthermore, at least one identical copy of Karl Weyland was in operation elsewhere at the same time.
Annalee Call was an Auton who successfully integrated herself into the crew of the mercenary cargo vessel the Betty, concealing her true identity as a synthetic. Her mission, apparently self-imposed, was to kill the Ripley clone aboard the USM Auriga before the United Systems Military could use her to clone the Xenomorph species. Call was later discovered to be a synthetic, despite the fact such units had been all but wiped out following a bloody rebellion many years previously.