My guess is that it's the built in infrared receiver. (Infrared: Nuvoton* Consumer Infrared (CIR)) If you are not using a remote control you can leave it as is.
A driver is available if needed within here http://tinyurl.com/qfzgz8t
This seems to be a common issue with the NUC. Our work has acquired 25+, and they all had this problem. In our particular case, we installed the combined WiFi/Bluetooth card, and it turned out that the WiFi driver did not get installed... our fault really, but easily missed, since, when you go to Intel's driver site, there are about four listings for network card drivers, and the two top ones (in our case) were the Bluetooth... only when we scrolled down some did we see there was separate one for WiFi.
In one other case (mine), where I installed a NUC as a HTPC, for whatever reason, the Nuvoton driver did not load (as mentioned by Stu.in.the.UK).
Easiest thing to do is go to Intel's download site, and download the "bundled" driver file, and just install them all (even if you don't need some). That seems to have covered everything for us.
you're correct. It was the CIR and after installing the driver the "unknown device" was gone. Thank you.