It's very common. If you see a guy carefully picking up his brass and putting it back in boxes or whatever, you can assume he saves it for his own use. But if he's just sweeping it up to toss in the trash, or walks off and leaves it, don't be afraid to ask, "You saving those?" or something of the kind. Almost everyone will give it to you if they don't reload or already save it for someone else who does.
Anything you find in the trash is fair game unless there's some kind of range rule to the contrary.