That LED is lit if the power cable is plugged in. It indicates that the Power Supply is at least providing standby power to the NUC. It didn't start two days ago (well, not unless you hooked up your NUC for the first time two days ago); it has always been on and you just didn't notice. To verify, shutdown your NUC and unplug the power cable. You should see this LED go out. Then, plug the power cable back in. You should see this LED come back on.
Note: Referring to your NUC using "Canyon" is insufficient. Almost every NUC has "Canyon" as part of its name. You need to say Hades Canyon or either NUC8i7HNK (or simply HN) or NUC8i7HVK (or simply HV).
I have this issue as well. Can this LED be disabled in BIOS ?
Also are some system components remaining powered on after shutdown to enable wake on LAN. wake on USB and standby settings or sleep, hibernate ect ? and can these be turned off in Bios and or Win 10 Power settings. When I shutdown I want a shutdown with no power drawing components or LED's still powered on. thanks