Not because the private key worked it doesn't necessarily mean you landed on a wallet that someone already owns. As long as you follow the right format of ethereum private keys then you will always successfully open a wallet. But then again, it doesn't mean that the wallet you opened is a wallet that someone owns.
I
could have some errors on my explanation, so to the readers: correct me if I'm wrong
If you want to dig deeper:
https://theethereum.wiki/w/index.php/Accounts,_Addresses,_Public_And_Private_Keys,_And_Tokens#Private_Key