Security by deterrence

When no one can watch (or trace one to what they do), the possibility of doing something bad (steal, break in, erase/modify/add data, etc.) is quite high. I was thinking about security cameras in supermarkets and shops. Is it more effective to have those cameras well visible to everyone or have them hidden? In my opinion they should be installed in visible locations; if they are not visible, there will be more bad people attempting to do bad things; it's true that you can track them down going through the surveillance videos and prosecute them - but think about the cost you have to incur; it is far more economical to indicate some sort of a warning signal. This will reduce the number such incidents and, yes, you can take necessary actions against those few bad incidents where bad guys dare to ignore the warning. Thinking in this line, you actually don't need real cameras installed all the places - you can safely have a few fake ones installed along with the real ones - they will act as a deterrence factor. (If you cannot afford to have a video surveillance system, it is at least good to have some fake cameras installed.)

What about firewalls, ID (Intrusion Detection) systems? I think we can make a similar argument about them.

Another side note, have you ever come across a situation where you cannot keep the lunch packets or any other food items from your co-workers in an office or classmates in a school? One crude way to do that is to take a bite and keep it :-) it'll surely act as a deterrence. A good way to have it packaged as if it's not a lunch packet - most hungry people won't bother to open that. This sandwich bag seems to be a good idea as well (though won't work after others figure out your trick)