Normally when an auction is sold, you can go to the auction's web page and see that it was purchased and by whom. Similarly, when an auction expires, you can see that it expired. And auctions that are still live have a "buy" button in place of that message, except if you own it, in which case there'll be a "cancel" button.
However, by my count, about 1-2% of all auctions disappear with no reason. When you go to the page of such an auction, any such message or button will simply be missing. Since this hasn't, as far as I can tell, affected one of my own auctions, I can't tell whether the system forgets the auction, or it doesn't remember what happened to it. I can, however, tell that this seems to predominantly affect auctions that disappear from the auctions list (as per the API) within about half an hour of being listed.
I have compiled a list of affected auctions if you want to see what I'm talking about:
http://glitch.erinye.com/test/badauctions.php