Just a thought but the intrusion switch could be bad. This support page has instructions on the location and replacement of that switch. I would open the case and check the switch to discover any problems. 
I have an Optiplex 7060 SFF (mini tower) with the BIOS udated at 1.14.   I had to check for the RAM model so I opened the cover/tower, and upon the next boot, the BIOS did not reported that the cover has been opened during POST, as I was expecting it. The feature is enabled is the BIOS and it was working OK previously. The only changed I made in the config of the BIOS, since the upgrade to 1.14  is to enable Trusted Execution for MVMM. Which seems unrelated to me.   Aynone can confirm this ? How do you report this to Dell ?
BIOS does not report opened cover on optiplex 7060 anymore