Hi,
I have used Siri when the phone is locked with passcode and tried to sent text messages and make calls and it never asked me or said anything that the phone is locked or something. It sents and makes calls even when the phone is locked with passcode and thus if someone finds your phone they can easily misuse the phone with the help of Siri even when the phone is locked with passcode.
So, keep your phone safe away from strangers.
I have used Siri when the phone is locked with passcode and tried to sent text messages and make calls and it never asked me or said anything that the phone is locked or something. It sents and makes calls even when the phone is locked with passcode and thus if someone finds your phone they can easily misuse the phone with the help of Siri even when the phone is locked with passcode.
So, keep your phone safe away from strangers.