Chassis intrusion detection usually relies on a physical switch which is thrown when the case is opened. Frankly, they're more often fitted on machines for business use than those for personal use (which doesn't mean to say you won't have had one fitted).
The switch would be a simple intrusion switch with a connection to the appropriate header (pins) on the motherboard. An example of one is as follows:
www.ebay.co.uk/itm/Genuine-Dell-0UC635-Chassis-Intrusion-Sensor-SFF-Dell-Optiplex-745-755-760-/252455452691
.....though design will vary, as will the positioning, but it should be obvious when the case is opened (if fitted).
That particular Asus motherboard has a header that can be disabled by means of a "jumper" - this is a small connector that simply fits over two of the pins, and shorts them out, or it can indeed be disabled via BIOS settings.
If it is a valid alarm, then neither the jumper has been fitted (or left in place) nor has the detection been disabled in BIOS. As I said above, it is by no means certain that yours didn't have intrusion detection, but I find it less likely on balance that it would.
I do wonder whether the CMOS battery comment might be relevant (as a low battery would cause a potential loss/non-memory of certain settings). In particular, the motherboard documentation contains "A message appears when you connect the sensor or switch for the first time or when you reconnect the sensor or switch to the chassis intrusion connector. Reset the system to exit the message".
You are obviously doing the above, and it is working at least once, but obviously being repeated.
I think if you're up for it there is a three-part action for this:
i) Open the case and check if there is in fact an intrusion detection switch (it will be intended to be held "off" by the part of the case you remove)
ii) Check the chassis intrusion header (to which the above should be wired if it exists) to see if there are loose wires, or if no switch exists, to check for existence of a jumper. If you do want to do this, it will be a visual check with the case open, and it should be possible to post a link to a diagram of where to check.
iii) Check the BIOS for the enabling/disabling of chassis intrusion checking. Bios is entered using "DEL" at startup on this board. I would suggest that you could do this check if you are careful (and change the setting if necessary), but you will need guiding through the pages. (It isn't under "Security" (no criticism of Mark - I looked there first - it's under some miscellaneous settings in the "Monitor" menu.)
If all of those look OK, then I'd go with the suggestion of changing the CMOS battery (which isn't difficult, but as above, the usual first sign of it being low is date/time issues).
Last edited by: tyrednemotional on Sat 13 Mar 21 at 17:55
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