From security point of view is it better to have camera hidden or visible
Hidden and visible cameras emphasize different security values.
Visible cameras provide deterrent value as much or more than recording value:
- They may cause less prepared or less dedicated criminals to think twice.
- They may encourage actions or routes which benefit the defender (e.g., walking around the visible camera field may force the attacker into the field of another, hidden, camera).
Visible cameras are more susceptible to avoidance or disabling, though, because they are obvious.
Hidden cameras provide improved recording value, in that they can be more survivable than visible cameras. However, they may have more limited fields of view, and they don't provide any deterrence.
The security decision to go with hidden, visible, or both, should be dictated by the site and the threat.
A convenience store is going to want to emphasize visible cameras, as deterrence is more valuable in that threat environment. A museum might emphasize hidden cameras, partially because deterrence is less of an issue and partially because obvious cameras detract from the atmosphere they want to provide for their customers.
In all cases, the DVR needs to be better protected than it was in this case. It should be protected well enough that legitimate employees can't tamper with it - certainly the attackers in the case you describe probably knew video stayed local to the site, knew it could be disrupted, and possibly even had "inside information" that allowed them to go straight to it.