In this day and age of extensive scouting protocols, huge departments to evaluate talent and dozens of outlets to view scouting reports it's hard to imagine there's such a thing as a 'sleeper' anymore, especially one at 324lbs in a size obsessed NFL. That being said, Amimi Solatolu out of Midwestern State fits the bill perfectly. You probably didn't get a chance to see him play last year, but if you did you would have seen a big, bruising, nasty offensive lineman who was raw in a lot of areas, but his run blocking was stellar.
Taking a small school guy is always a risk, as it can often be hard to evaluate a player's ability when he's facing off against inferior talent, but that's the risk a lot of teams will be willing to take. The key for me is looking intently at his intangibles, and seeing if you have a player willing to rest on his laurels, or someone who will put forth the effort at the next level.
Plays with a nasty streak, loves to finish and his combination of range/athleticism and balance makes him dominant when trying to kick out defenders. Loves to block down field as well, routinely plays till the whistle and his range in space is very impressive, especially the way he covers ground and breaks down so quickly, dropping his pad level and/or cutting defenders in the process.
The NFP see this quality out of Solatolu too. I've seen reports that some scouts believe he's 1st round talent, but wouldn't dare taking him that high (presumably because they think he's a sleeper). If getting a big, bruising run blocker is in the cards for the Falcons I think they could do a lot worse than Solatolu- especially if he finds his way to the 3rd round. At that stage he's an absolute steal, and looks like a player who could be a 10 year starter with a little TLC.