I remember having a mobo which did the same thing. I only noticed it after about 5 months of owning the board becuase my
PSU DC rails were low, and I was trouble shooting seeking any anomalies. I had thought there was a short in my system. I RMA'd, the board, got one brand new and this did it to. I even had the reseller checking their stock. Come to find out, it's true about 90% mainboards do this. As mentioned above, I believe it does have something to do with the "Wake-on" device which can be KB, mouse, Lan, Modem, etc.