Once you’ve logged into FaceTime you can have a look at all the account settings of the used Apple ID. Username, ID, place and birth date are shown as well as the security question and the answer to it – in plain text, without another password request.
Also, when logging out of FaceTime, the password field will still be filled in. Moral of the story: do not use FaceTime on any Mac that is not your personal, password-protected machine. Keep in mind that FaceTime for Mac is technically in beta, so bugs (yes, even security ones) are likely to exist in the software. Apple will probably release an update that fixes this in the near future.