See, I don't understand this, "they gave up" justification. Nowhere else in the history of humanity is this applied. I've never found something with someone's info on it and thought, "Gee, I wonder if they want this back." Of course they want it back, they put their info on it, you frozen stick of poo. If you find a missing child after she's been missing for 3 years would you not bother telling anyone b/c the police and parents stopped appearing on Nancy Grace?
Or let me put it to you this way, do you know what happens when a shipwreck is discovered? Every nation that has ever used the area the wreck is found becomes extremely interested in the nationality of the ship, regardless of what's on it. You can't say, "I found the shipwreck and its historical items so I can keep it b/c Spain gave up looking for it," b/c Spain will sue your ass.
Now, I don't really care b/c at the end of the day it's just a piece of plastic but by the same token you shouldn't get your jollies by ganking every disc you see. What do you do with 50+ found discs? Melt them down into a plastic love doll because you can't meet real girls because they think you're a stingy little miser? At the end of the day, just don't be a douchetard. For the sake of humanity, any little thing you can do to make living on this rock a little better for everyone, from not littering to making a compliment to returning a disc, makes you a living saint.
If you don't return discs you are a complete and utter douchetard. It's that simple. You're a selfish little man surrounded by tainted, worthless plastic that won't fit in your coffin. If that's your modus operandi then I hope you enjoy being a douchetard b/c that's all you are.
Sorry for the rant, Three Putt