The front panel has usb 3.0/2.0, and audio and power/led. Pretty basic.
Op, there's only one solution, you'll have to chase that wire down, find out exactly where it does come from. Usb3 is 19/20 pin double row, long rectangular block, USB2 is 9/10 pin double row, short rectangular block. Should have 1 of each. The power/leds should be singular pins, and the HD audio will be the longest wire and looks similar to USB2.