I remember reading somewhere about Techpriests who preferred to keep their augmentations subtle, so as not to terrify the initiated. Sort of mechanical missionaries/spies and spies. I've talked to several of my friends and they swear it sounds familiar, but I can't find a source for it anywhere.

Factor of the Lathes, Lathe Worlds p45. Fabricated Flesh (Talent)?

The first time I heard about these guys was in the novel "Mechanicum".  Not sure if they called Factors there, but one of the main characters was a diplomat for the Mechanicus.  Factors are not so much spies as diplomats.  Not that the Lords Dragon wouldn't be above asking them to commit industrial espionage…

The novel, Priests of Mars, features a female Magos that has subtle implants and appears fairly 'normal' at first glance. She wasn't intended to be a Factor/diplomat/spy - just a bog-standard Magos with a specialty in (IIRC) mathematics/stellar navigation.

Good and best quality implants/augmetics can be subtle if you want. This is stated in the main book. No need for specialist careers really. Just most of the mars priests see no reason to simulate fresh. Some do.

It's less that I was going for a specialist Career and more an official name for these guys, so I could say something like "Oh, well, my Explorator is part of the Androvisage sect" and have people know what I meant without having to explain every time.