generally the only leds on the rear of the case are...
-psu leds (this oviously isnt that)
-sometimes a led on buttons high end motherboards have back there (obviously not that either)
looking up the issue reveals that the case has a led back there so that it lights up the area so you can see to plug in cables (in the dark?) which is pretty harmless (and almost useless)
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if you want to disable this... you can probably unplug the molex (or whichever other type of power connector) goes to the switch on the top of the case which controls the led. disabling the power tot he switch would disable the led.