This is the same issue with Siri bug just discovered recently. Without unlock the phone, I can activate Cortana to call my wife.
In order to stop this, I have to go into Cortana settings. Under speech setting, you have to uncheck 'Allow speech above lock'. Hope this is something that Microsoft can fix before release WP8.1.
In order to stop this, I have to go into Cortana settings. Under speech setting, you have to uncheck 'Allow speech above lock'. Hope this is something that Microsoft can fix before release WP8.1.