I fast traveled into Sanctuary and upon arriving I hear a fusillade of turret fire. So, I look around and.... no enemies!
I've placed my fast travel target by the circular drive with the big tree, which I have completely surrounded with platforms with about a dozen turrets. The lone guard posted at the north end by the bridge from Vault 111 is trying to hide behind her guard post, to no avail. So many turrets, so many lines of fire, she's dead in less than three seconds.
Sure enough there's a synth component, but it surprised me that it was the turrets that took her out and not her fellow settlers. None of my other settlers are anywhere near that cul-de-sac.