There's obviously a difference between finding something and stealing something.  But there're considerations that should be accounted for regarding the particular item found.  If I find a brick of gold that had been buried on my property which I've owned for many years, I'd consider that brick of gold mine, all mine.  But if instead, I found a piece of paper with a bank account number, username, and password, I would shred it.  There's obviously a difference between those items.

Of course I understand there's a difference between blockchain and bank accounts, but in this case it's impossible to know whether the owner has a back up or not.  Assuming failure of any attempt to inform the owner of the breach, I'm of the opinion that the ethical thing to do is assume the owner does indeed have a backup, and leave the funds untouched.

Now having said that; I understand there's little overlap in mathematics and ethics, and I value the freedom blockchain has given us to choose.  But, that's freedom for you.  We all get the opportunity to choose how we behave, and the right choice isn't always clear.