Well as soon as I got my question typed in I figured it out, at least for this Intel motherboard.
I found another computer with the same motherboard and browsed the BIOS settings. Under Boot/Fast Boot/USB Optimization I found this descriptive text (nowhere in the manual by the way):
"If Enabled, USB devices (keyboards and drives) will not be available until after OS boot, but BIOS will boot faster. If Disabled, USB devices will be available before OS boot, but BIOS will boot slower. This question does not affect USB capabilities after OS boot. This question cannot be enabled while a User Password or Hard Drive Password is installed. In order to disable Fast Boot without entering BIOS Setup: Power down the system, then hold down the power button until the system beeps."
I tried it, heard 3 beeps, then it booted to a screen saying that the Fast-boot trigger had been detected, would I like to enable fast-boot on the next boot. No!! Noticed that my keyboard light was on and hit 'N'. It rebooted again and I was able to hit F2 to get into the BIOS.
Sometimes my Nuc doesn't produce a beep, so I count 3 seconds while pressing. – Rael Gugelmin Cunha – 2018-04-18T19:12:34.570
2My question is why would anyone want this enabled in the first place. Seems to be a security feature to keep users out of the bios. Thanks for the info, screenshot of the bios page and description would be nice – Moab – 2011-07-20T23:36:08.080
2Maybe if you're really concerned with bootup speed, but definitely not a good default setting. – jacobsee – 2011-07-20T23:47:41.227