If you look at the full card list for Magic 2015, at Gatherer or elsewhere, you'll see a bunch of cards listed with collectors number 270-284. What are these cards, and why are they listed as being part of Magic 2015? I played enough M15 Limited to be pretty sure that they aren't actually present in packs, and they're listed after the basic lands, which is very unusual. Is there some secret, bonus release of M15 that had these extra cards?
You are correct, these cards are not in booster packs. For the past few years, Wizards has put out 30 card sample decks to introduce new players into the game. This year they decided to put into these packs a number of reprinted cards not actually present in M15: see here for decklists. Like the article mentions, the Deck Builder's Toolkit for M15 also has the extra cards.
Since putting two of these sample decks together gives a basic 60 card, two-color deck, Wizards expects a number of people to do just that to build their first standard deck. They made those extra cards explicitly standard legal to allow people that option; most people would assume by default that such a deck is playable.
These are cards which you can get in the M15 learn to play packs. You can get these at your local (WPN) game store, but are mostly given to players which are new to the game.