Although I have a feeling that you have looked into this already, i'll humor you because I am have some spare time at the moment (procrastinating actual work):bored:.
I found a great deal of info pertaining to deleting CCTV via google search, and I found an interesting website from a CCTV company in the UK... humor me here.
"The quickest and most convenient way to erase a CCTV recording on a tape is by using a bulk tape eraser. Bulk erasing a CCTV tape can take just 5 seconds, returning the tape back to its original condition, completely clear of any previous recordings. This also ensure the CCTV tape provides the best recording quality (http://www.veritysystems.com/degaussers/erase-cctv.html)". I found this interesting for 2 reasons. 1) I had no idea it was that simple/fast to erase info from CCTV, and 2) the prior paragraph, which discusses admissibility of CCTV footage in court, "Under new rules and guidelines relating to the use of CCTV video recordings being submissable as evidence, state that it must be proven that a video tape has been completely erased before being reused if it is to be submitted as evidence".
Also, here in the states, it is common for CCTV to be set up with an accompanying USB drive. CCTV Camera Pros recommends using a USB thumb drive because you can backup a lot more video to a 4, 8, or 16 GB USB drive than you can a 700MB CD-R disc. The purpose of the backup system is to allow the DVR operator to export a specific segment of video so that the video can easily be transferred to the police in the event that something occurs that needs to be investigated. The file that is exported by the backup process can be replayed using the video player application that is built into the integrated remote station software that the DVR comes with. The backup system is designed to export minutes worth of video. The system is NOT meant to be an archiving service to export hours and days of video to an external drive (
http://www.cctvcamerapros.com/DVR-Video-Backup-s/263.htm). I am reading this as to mean that there could be more than one form of the CCTV footage, possibly on UDB, and also that if someone is tech-savvy, it would not be hard nor would it take long to delete something from the CCTV. This is all the info I could dig up in the 15 minutes I have right now to look into this, but I know that the missing footage has been eating at some of us, so I wanted to see what I could find. All :cow: