Later, in Devarim (22:3), the Torah explains that you should return any lost item that you find, not just a lost animal, and not just if the owner is your enemy.

The Mishnah helps us understand when to perform hashavat aveidah. The main question is: will the owner still be likely to hope to get their item back? If you find something that has identifying signs (like a name on it, or if it has an unusual shape, or if it has unique markings, or if it was placed in a very particular position), then you can assume the owner is hoping it will be returned. You should do whatever you can to try to find the owner.

But if the item you find has no signs, like if it’s a dollar bill on the street (that looks like every other dollar bill in the world, and could have come from anyone), then you can assume the owner probably gives up, so you don't have to return that (Mishnah Bava Metzia 2:1-2).