In mist you create an account and you provide password. Now your keystore files have been accessed by unauthorized people but not your password which is in your head and only there. Can your ethers be stolen using only those files?
"Unlocking" you wallet if someone can have access to your wallet file depends of your password strength. If you really want to store all your assets in this wallet, please, keep your wallet files and password safe on an encrypted offline computer or use a hardware wallet like Ledger nano s or Trezor. Trust no one.
Even if your passport is safe, it can be guessed by simply trying all the possible passwords. For your password to be secure, it has not only to be long, but to be somehow random.
password can be guessed easily. If you use real words, you have to use several of them, chosen randomly. To learn more, you can search "entropy in information theory".
It is always easier to attack your password than your secret key, so you should really pay attention to get it secure.
Any password an be guessed, but the cost becomes exponentially higher with entropy (i.e. "randomness + length").