You're probably right, here a higher resolution pic:If I were to guess: a small switch in that new hole directly above the motherboard I/O (the slightly larger one with screw mounting holes on each side).
A mistake IMO, but then I guess people don't really use power buttons all that often, so it might not matter. Still an odd decision: more front I/O is a very "for the mainstream" kind of move, while moving the power button to the back where you can't see it or even tell that it's there unless you know what to look for is a very "SFF enthusiast" kind of move. Trying to please everyone, and thus not pleasing anyone?