If a thief gets hold of your iMac and TC, and he gains access to the iMac, he can steal everything very easily. All he has to do is reset the keychain and all your passwords are gone. If you're worried about that sort of thing, you should setup a firmware password on the iMac and use FileVault encryption. Then, if the iMac is stolen and even if the hard drive should be removed, the data would be inaccessible. Kind of drastic protection to use, but if it is something that concerns you.....