That would require manual intervention which is insane. It would also not stop anyone who has access to the nvr to removing the privacy mask. He needs to secure the NVR and just let it record. Or use a vms that allows disabling a camera by stopping display and/or recording easily. Blue iris can do this no problem. Like every NVR you must secure it or anyone can turn the cams back on.
It sure does exist, you simply choose not to use it. The alarm can be easily disabled if someone was determined. Nothing is certain. You cannot guarantee security unless you sit at your kitchen table all day with a revolver and never leave your homestead.
I thought he wanted to do it remotely, in case he forgets. I don't know if the app allows to turn privacy masks on and off. I think it does.
Even if it allowed you to do it going through the app, enabling or disabling several cameras every time you leave or return is insane.. It's not a practical or even usable solution..
