If this happens after powering-OFF and maybe only within a 10min. framework or less, then it's bleeding any residual power form any source that captures it. If at far longer period of time, then its something to do with household current. Yes, household circuits can be found to make circuit that's low voltage and as USB runs on 5V and the LED themselves only flash, they can glow at lower voltage for split second. Overall, I wouldn't worry about this. However, sometimes the quality of devices and/or make-up of the overall setup shows itself in some ways undesirable.
Once you shutdown, pull the AC cord at the PC and see how it reacts, then. This test may localize source of voltage(s) at PC or household(or remaining items attached).