ATL Delta lost and found is behind the oversize bag carrousel --- against the wall to the left of the doors going out to the taxi stand. You go there to look for things left on planes, or somewhere that a Delta employee might have found your item.

ATL airport lost and found is on the north terminal side of security (past the First Class security line), up an elevator.

In my experience, if you left something on a DL plane, you go to the Delta LNF.

If you left something in the terminal, even perhaps a DL gate, you have to visit both of these LNF's because who knows who (DL employee or airport employee or even another passenger) might have found it and turned it in?

For my friend's lost laptop, of course traipsing to both LNFs multiple times was an futile effort

And yes, you have to describe the item to the front person, who will 99% of the time spend a couple minutes in back before coming back and saying "sorry, it's not here".