Why does my mouse's LED not turn off when I shut down the PC?

Why doesn't my optical mouse's red LED turn off after I shut down my PC?

The optical mouse's LED stays on because the mouse itself is still being powered.

Just because the computer is turned off it doesn't necessarily mean the USB power is turned off. This is done so that the Power key on some keyboards can be used to turn the computer on.

And why does it stay red? Simple. It is red. Why does the grass stay green...?

Your laptop or PC might have a USB port that supplies power(to charge phones or power devices like a battery bank) to it even though the system is off

I know this question is old, but if someone else(like me for the past few weeks) is looking for a solution to this problem, maybe he/she finds this helpful.

The following solution worked in my case (credits go to Reddit user /u/zelloxy) :

"Start command prompt. Type powercfg -devicequery wake_armed. Enter powercfg -devicedisablewake <deviceName>. If that does not solve it try the next one. In my case, my mouse was typed as a keyboard even though it was a mouse. At last it worked."