In regards to Wesley. The process of the computer tracking a specific person throughout the ship could be circumvented by someone with appropriate security clearance; such as Commander Data; ordering the computer to not track Wesley. Likewise with Ben Finney. He not only had the ability and clearance to alter the video logs. He had the security access to keep the computer from tracking him. The computer doesn't need to detect life signs in order to visually track a person moving throughout the ship.
One known aspect of starship computer systems is the Intruder Detection System. Which has been mentioned as early as TOS. It seems the ability to detect unauthorized lifeforms through either visual or other means of shipwide surveilance is mandatory for the system to work. The system only doesn't work when the plot requires it too.