The definition of theft is to take something with the intent to not return it.

Once you found the backpack, you would have to return it to the thief for it not to be theft.

Hopefully you wouldn't get prosecuted, but you are then at the whim of the prosecutor. If the thief calls it in you've created a bad situation. I was in jail and saw weird crimes like this. A guy bought some dope for $40 from a dealer, but only handed him $20 and ran off with the dope. The dealer called 911 and reported the stolen merch. The buyer was in jail with me. WTF.