Quite curious as to what drivers Windows installed on its own. I have the D54250WYKH and all the drivers needed to be installed manually by downloading from Intel's download section.
Saying that, you have the 7260 Wireless N wireless card. Like the NUC drivers which require you to manually download drivers (from another net-connected computer), said wireless card requires drivers from Intel's site.
Get the right driver (Windows 7 in your case). The latest version of the 7260 series cards is 17.0.5.
EDIT - Just re-read your question. Where was the card purchased from? The really curious thing is the machine actually sees the card without issue (albeit currently in the 'unknown' device in Device Manager). So it doesn't seem to be an issue with the PCI slot at least. I would have some doubts about the 7260 card as there are some counterfeits around, but if you purchased from someone legit then that's ruled out too.
Have you tried rebooting after each driver install? Do you have ALL the NUC drivers installed?
The card appears to be properly inserted into the slot from such angle. Is the adapter recognized in BIOS (Devices, PCI)? If not, try a BIOS recovery.
Perhaps you may want to try powering the system off and remove the power cord before turning on again. This has helped for a different issue.
Otherwise, I guess either one of them is not working properly.
Yes, it was in the end a driver issue.
On this page
I eventually downloaded Wireless_17.0.5_all.zip
And in that zip file launched the setup.exe in the Wireless_17.0.5_Win32 folder
This solved the problem.
I think that Intel should improve the documentation / instructions:
1. Include that software in the driver bundle installed (1st July 2014) WYK-WYB-Win7-32bit-Driver-Bundle-Jul2014 so that user's don't have to search here and there
2. Make it clear that the "for IT administrators" version should be used. I initially downloaded the other one (for users) and for some reason, it did not work.
Anyway, both my wireless card and my NUC are good, that's good news.
One more thing. The intel driver update utility shows an available update for Bluetooth.
I have 17.0.1405.0456 installed.
However, when I click on download now from the update utility, it downloads BT_17.0.1405.02_s32.exe This is exactly the same file that I used yesterday to get to 17.0.1405.456.
Is the update utility pointing to the wrong file? When I launch it, it gives me the only option to repair or uninstall.