You can only think of deleting the CCTV footage if you are the owner of the camera, since you have to put in the admin password in the SD card to remove it; an intruder would not know that. In the modern era. The SSL encryption secures the CCTV footage, and because of the authentication that is required, if it is not given, the cameras can send email alerts to the owners and make them suspicious in case of theft of the footage as well.