[Groxx]

Why wouldn't you have prior knowledge? You presumably know where it goes - just look at the keyhole. Or if not, a single identifiable object in the photo gives you a lot of scale information. I doubt they're as concerned about white-background newspaper-worthy shots like the NY keys, and more about shots at a distance (keys on a table, for instance).

edit: http://www.jacobsschool.ucsd.edu/news/news_releases/release.... for an example