System found unauthorized changes on the firmware

I have a PC with an Asus motherboard running Windows 7 Pro. Yesterday I installed latest Windows updates, restarted, and shortly afterwards hibeated the machine.

This moing on un-hibeating I got a black screen with:

The system found unauthorized changes on the firmware,operating system or UEFI drivers

It also said:

Go to BIOS Setup > Advanced > Boot and change the current boot device into other secured boot devices

The only way I was able to get it to boot was by changing the BIOS secure boot OS to 'other OS' i.e. I think this means secure boot is effectively disabled.

There have been no hardware changes to this machine recently (last thing I changed was adding more RAM about 3 weeks ago, and it has successfully rebooted multiple times since then). There is no CD/DVD or bootable USB device connected.

I'm running a full MSE scan just to be sure / paranoid. Is it possible the Windows updates caused this? Should I be conceed about disabling secure boot? I've tried setting it back to enable however the original message reappears.