In order for a Space Marine to become a techmarine, they must spend time training under the Mechanicum on Mars. This doesn't seem to be much of an issue for most Chapters, but what about those who don't have easy access to shipping lanes, or those who don't want their presence known?
One example is the Carcharodons Astra (or Space Sharks), who, according to Robbie MacNiven's novel Red Tithe, have effectively been exiled from the known galaxy, only appearing periodically to raid outlying colonies for supplies. According to sourcebooks, they have techmarines but they usually remain on their fleet due to their scarcity and importance, but you'd think that they would eventually need additional/replacement techmarines, even given the extended lifespan of a Space Marine.
Another example would be the Grey Knights. While they are practically neighbours of the Mechanicum, the Chapter is a secret to the average mortal, and even among other Chapters, they are thought of as a legend by most. So how do they get techmarines? They certainly have/need them, and according to a Wiki, they're trained on Mars like any other Marine. But do they arrive disguised in the livery of another chapter (or do the Grey Knights have a public-facing "front" Chapter) for such purposes? Or does the Mechanicum have special facilities to train them secretly?
If someone could point me to a novel our sourcebook to explain how these Chapters (the Grey Knights in particular) train their Techmarines, I'd love to see it. Thanks.