How does a motherboard detect case open/chassis intrusion?

I've always noticed I have a Case Open detection in the firmware settings of my motherboard, but it have never returned a true value, instead it always says that it's not open, even if it is. I am very curious as to how this function is supposed to work.

Its not complicated. Generally, there will be a microswitch on the case. When the case is opened, the switch is released. This switch is attached to the motherboard directly or by wires. The current is supplied by the CMOS battery, so it will work if the computer is unplugged.

Manufacturers typically do not put the switches in consumer or low end PCs. Switches and wires cost money. And the general public does not open their PCs. The manufacturer would rather save a few cents on 10s or 100s of thousands of PCs that they build by not including it. That and the extra cost doesnt offset the cost of consumers calling in asking why they see "Intrusion Detected" when their computer boots.

As you said, it is in your BIOS, but you doesnt report if it is open. The motherboard manufacturer isnt going to have a custom BIOS made just to remove that feature. Your motherboard may even have a pin header for the intrusion detection switch. You might have to consult your manual. In theory, if its there, you could rig one up.

You arent going to steal memory out of your computer, which you purchased and have at your home. This feature is targeted to organizations where employees with sticky fingers are far more likely to steal components like RAM out of a computer. However, intrusion detection in this matter might not indicate what was stolen, or even if anything was. Also, if you arent looking at the computer when it boots, you wouldnt know. Nowadays organizations use software to monitor if the hardware has changed on a computer.