Well, you're in luck, as I happen to own this exact model, which has the same hole.

Taking it apart is quite easy, just 6 screws and some plastic tabs hold it together.

Here's the components on the underside of the board beneath the hole (these components face the rear backplate)

And if I pry the case apart a bit, I can't see any components directly beneath the hole, it's just bare PCB.

As far as I can tell, no microphone is present. The hole is likely a product of the injection molding process for the outer case.
One thing to note about this device though; while the ergonomics are great and its been reliable, I notice that my left hand wrapping around the case can sorta block the mouse pad signal to the dongle (about 6 feet away), so I need to hold it in a way that the transmitter isn't blocked by my hand to move the mouse curser smoothly.