This.
I was wondering why would it need a rescan. I thought the full blockchain is needed only for the transaction history.
It can't know the current balance / past transactions of a new address. When you import a private key via the console it will start re-scanning the whole chain until it finds the relevant information.
In case that someone doesn't care about the history, why would he just import an address of private key to check balance?
Computing the public key / address and checking via a blockchain explorer is the faster way. I'm not sure if there's an easy to use tool for that.