Q: After playing tennis in the local JCC, my friend left some balls on the court and went home. Am I required to return the balls to him?

A: The Gemara (B.M. 25b; 31a) teaches that when someone is careless with his property and knowingly leaves it in an insecure place, it is called aveidah midaas (knowing loss) and you are not required to do hashavas aveidah (C.M. 261:4).

Nonetheless, the Shulchan Aruch writes that the item does not become hefker (ownerless). Although you are not required to collect and return the balls, you may not take them for yourself. The Tur and Rema write, on the other hand, that a person who recklessly abandons his items renders them hefker and you may take them.

Machane Ephraim (Hil. Zechiyah Mihefker #6) explains that this depends on the evaluation of the circumstances. For example, if the balls were old and worn, they certainly would be hefker, whereas balls in good condition would depend on this dispute (see Pischei Choshen, Aveidah 4:12).

If the balls were not abandoned, even if not left securely, e.g. a can of balls was left in the locker room, they do not become hefker.