But since there's no way to know who they belong to, keep them for yourself and / or sell them.
This sounds horribly unethical. The coins don't belong to the OP. The original owner might have a backup and might try to move the funds once they notice that their paper wallet is missing.
Technically, whoever owns the private keys, owns the coins.
I agree that it may be unethical to 'take' them if we assume whoever lost that seed has another copy and might notice.
I don't know the circumstances, e.g. if it looks like it is very old (so owner should have noticed by now) from the exterior, when the funded addresses were funded and details like that.
Maybe it would be sensible holding onto the seed and only 'taking' (spending / selling) the coins when some more time has passed.
Anything I describe though, will be better than either destroying the seed or giving it to the police.