I have the same NUC and had the same issue. I'd walk through the bedroom and it would mysteriously be on all by itself. I'd shut it down, then a little while later it would be on again. A couple of weeks ago I updated to 27 and haven't seen the problem since. I'm running Openelec on mine. I should mention right after I updated the bios I did a clean install of Openelec...not because of a problem, just because I got sick of Windows.
I also noticed (or I'm just imagining it) that the fan seems quieter after the bios update. Weird, huh?
Hope it works for you!
Well, unfortunately, my problem only started to happen with 27
Do you have Wake on LAN enabled in the bios?
I've tried with Ethernet unplugged and still wakes up.
Tried also with HDMI (because of CEC) and still wakes up too.
I would still check the settings in BIOS. Even with no ethernet plugged in it was happening to me. I've seen that on several systems, even my laptop.
I set the setting as suggested.
I will see tomorrow morning if this was the solution.
After a night it seems OK.
It stills powers with CEC but that is a totally different problem which I may solve elsewhere.
Anyway, it is a bit odd that without LAN, WOL is powering up the NUC.
Many thanks to you
Glad to help! Of course this isn't technically a "solution", it's a workaround because it requires you to disable a feature that should work. Still, if you don't need the WOL feature this should do it.