A sleeper should not be on your immediate radar. A sleeper should be, by definition, tucked away under a serious blanket of almost complete anonymity. If you have heard his name more than once, chances are he's not a sleeper.
If your buddy tells you, "keep an eye on Brian Price of UCLA. He's a beast"! Ummm, that's not a sleeper.
However, if the guy eating peanuts at the bar says to you, "Ya know, I think that kid Porter from La. Tech could be the next Willie Parker," and you find yourself saying "Who?" Then that is a sleeper.
These guys may or may not be at the NFL combine. They may or may not be called on the first day of the draft, but eventually, you will know their names. And when that happens, look out—because you may very well be a witness to history in the making.