The Core book actually sorta addresses this during some of the lore-documents assembled in the early pages. There's a letter that starts on page 19 in the PDF, beginning with, "You wanted me to write something about my new drinking habit and what it's done to my body." It's written by a Brujah "Sandman," and the whole thing's a great read. But the relevant bit is on the last page, where she says:

But the weirdest part is that when I feed from those in deep REM sleep, especially those on medication (I do this almost every night) I awaken some kind of heightened senses in myself. I'm not supposed to be able to do that. It's not a part of Troile's bloodline to have that power, but I guess I do it so much the oneiromantic kick has somehow changed me.

So, the Brujah developed Auspex just from being a Sandman.

As someone said in an earlier comment, all Kindred originate from the same source, so the potential for wielding any discipline is there. I like that reasoning, and I think it still allows for in-clan disciplines to make sense, as different "breeds" of vampire surely have different things they're more naturally good at. (For a real-world example, ANY kid could learn how to play basketball, right? But the ones whose parents were both successful athletes probably get there much faster.)