Yes, usb power may be supplied via +5VSB (5V Standby power), even when the PC
is powered down. This is in order to support "Power On By Keyboard" (when one is using a USB keyboard).
All Mobos i came across that support that feature had some jumper(s) to select wether power is supplied by +5VSB or by the normal +5V rail.
if that power is a concrn to you, RTFM to find this jumper.
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