I don't know if you've seen the movie or not. I can easily see how they missed it. What you see is a far-away line of dust at the upper-right hand corner of the screen, which is as far away from the action as it's possible to be. In addition, there's a chimney with smoke in that section of the screen, and you'd have to look closely to realize that this wasn't just chimney smoke itself. I'd never noticed it at all if I didn't hang around the no-life fanboy sites where someone had posted about it. As it was, I'd seen the movie two times before and hadn't noticed it; my friend the Texan Elf and I watch the movie together and helped each other spot it.
MWUFFLTHBH swears it's a black rider on a far distant road in the background. Who knows ... he could be right.