I read this a few weeks ago- during the first hellatus, I think. I lost track of it. I know it was gen, and a one-shot set in season 5.

Sam and Dean were in a grocery story or Walmart, and Dean watches Sam help a little girl find her mom. Dean muses on how Sam is nothing at all like Lucifer's vessel should be. At the end, Dean says that Sam did good with the kid, or something like that.

This ringing any bells?

FOUND! (In record time, too) link in comments.