What would you do if you managed to come across someone else's private key and it's worth an amount that most people would 'prefer not to lose', assuming you have no [idea who it belongs] to or [location clues]? Eg. Sweeping through common books' text for BIP39-based keys.
The place you found the private key matters and can give an idea of who may have it and what to do with it. If it was found on a road like a lonely street, trying to find the identity of the owner will be much more difficult than when you found it in someone's house or private space. If you found it in someone's house and you still even went ahead to check the balance, that's criminal to me, but if you somehow found it on a lonely street, on a public bus, in the park or any public place where it can belong to just any individual, it will not be a crime to know what the seed phrase is worth, not to take it but to be conscious. The owner of the seed phrase will most likely know that they have lost their seed phrase and make efforts to try to get to where they kept the back-up to move their money away, so your duty is just to keep the seed phrase and give the owner some time to move his money out.
Note: By keeping it, you have prevented the seed phrase from getting into the hands of people who do not have good intentions and may just want to withdraw everything to another place before the owner can do so.
In the case where it seems like it is something that the owner has completely lost, it is just too sad, but giving it to charity becomes the next best option.