The Heretek background gets the Master of Hidden Lores bonus which states you get a +20 for Tech-Use tests to comprehend, repair, use, or modify an unfamiliar device if I have a relevant Forbidden Lore skill at Rank 1 (Known) or higher. Is there anything stating whether you're familiar or not, or is it all up to the GM and circumstances in this case? RAW, errata, and dev comments are all welcome. If none of those are available, a widely accepted ruling might be a useful foundation to stand upon when discussing this with my GM.
There is no errata or dev comments that I have been able to find, no mention in the FAQ's and I'm not sure a RAW/RAI reading even applies. Using this background bonus has to be a discussion between the player(s) and the GM on a case by case basis since it can easily be abused by a powergamer.
Let's take an examples:
You have Forbidden lore "Craftworld Eldar", your team has somehow gotten stuck on a derelict eldar cruiser overrun by some other Xenos species and you are trying to escape. In such a situation you would get your Master of Hidden Lores +20 bonus to do just about anything tech related onboard.....IF and ONLY IF you unfamiliar with the item.
Have you done it before you don't get the +20 (before as in earlier in your career, not as in "you just opened a bulkhead door, now all the rest are +0").
Think of it a bit like you obsess over new things to such an extent that you "just have to make it work", and once the initial high of trying new tech then it seems boring and useless and not worth the effort to work with - so normal stats apply.
@Popperlicious gave a good answer here with regards to the example of a Heretek with Forbidden Knowledge (Craftworld Eldar) getting the +20 bonus to Tech-Use to go activate an unfamiliar device on an Eldar ship.
Here's the text from Dark Heresy 2e - Enemies Without page 32:
Master of Hidden Lores: When a Heretek makes a Tech-Use test to comprehend, use, repair, or modify an unfamiliar device, he gains a +20 bonus if he has one or more relevant Forbidden Lore skill specialisations at Rank 1 (Known) or higher.
I figure whenever any character is taking a Tech-Use test to "comprehend, use, repair, or modify an unfamiliar device," they'll have some sort of penalty for it being unfamiliar. Based on the fact that the bonus comes from having an associated Forbidden Lore, it makes sense that the device would be esoteric or alien to the layman by definition.
Rather than construing this bonus as representative of a Heretek's drive to and obsession of learning new things causing them to spontaneously solve a novel problem, I'd say that to me it represents a Heretek having historic, forbidden knowledge of a topic and being able to at the very least tangentially relate it to the current situation enough to get a bonus - in essence, their worst case educated guess is still pretty good because they've seen it once or twice before and possibly studied it.
And of course if the Heretek passes the test to "comprehend, use, repair, or modify an unfamiliar device," then I'd say it's no longer unfamiliar after that, eh?