I Have the same issue now in 2015 has any one worked out what that wire was for ?
It's a grounding wire. Just secure it with a screw in one of the holes right underneath on the metal frame.
Also holy bump!
I actually found a few pics I took way back when, when I first encountered this issue.
IIRC, I ended up mounting it to the hole where the red arrow is. I believe I first initially mounted it to the 5.25 drive bay seen in the 2nd pic, but if I recall it didn't leave much slack when snapping the panel back on.