|Chassis intrusion simply means that the case has been opened while there is a chassis lock switch connected to the motherboard and enabled.|
You should be able to go into BIOS and disable the alert.
This feature is common on corporate environments as a means of detecting & deterring users messing about ( stealing ram & hard drives), or more seriously, temporarily removing hard drives in order to copy the contents.
You haven't said but if this is your home computer I would bet it is an ex-government or corporate unit bought second-hand. Ignore the alert or turn off the thing in bios. You will need the admin password in order to do so.
Goin' Fishin' (Some day)