Are you absolutely sure that all those wallets aren't just password-protected with no receiving addresses created for it yet? 

Core doesn't create receiving addresses for new wallets by default, you have to explicitly tell it to give you an address so it's possible the wallets you're trying to dump were only created but never used (by virtue of having not receiving addresses for it).

As far as looking for the private key is concerned, look for something with "5H" "5J" "5K" or "5L" at the beginning of a line.