Regardless of how you acquire the seedphrase or access to the wallet, the fact that it's not yours, then it's not yours. Setting the law aside, we know whether it's right or wrong so it depends on one's conscience. But given what op said, let's say you luckily guessed the key, the first question would be, why are you trying to access other's wallet? What's the reason behind it? That part alone means you already have a bad intention. On the other hand, let's say that you just accidentally found a paper or anything with its seedphrase written on it... Let's not lie, we'll all be tempted by it especially that amount can already change your life in an instant. So I'd probably monitor it for some time to see if it's really a dormant one before doing anything.
You will monitor it for sometime before doing what exactly? I believe that's the question that you are been asked, for anyone who guessed someone's recovery seed they are already planning to steal from the wallet if they find anything inside it, if I ever mistakenly come across such big balance I am moving some money from the wallet.
If you have a billion dollars and you lost a thousand dollars you won't feel anything, this is what I am going to do not because I need that money but to send a message across, that the wallet isn't safe anymore, Bitcoin wallet has no messaging app where you can PM someone via decentralised network to warn them, the only way they will know that something is going on is you sending some money from the wallet.
