Definitely a microphone, and almost positive its a glass break sensor, but that's a huge one! Sucks that they screwed it into the plastic, if you ever remove it, you'll have a hold in the panel.
Easy way to tell if its a glass break sensor. Roll rear window down, arm alarm, wait for 30-60 seconds, then snap your finger next to the mic, alarm should go off without much noise.
I'd be willing to bet you also have a non-oem alarm keyfob too. If not, see if there's an alarm siren under the hood somewhere, that will tell you whether you bought a vehicle that had an alarm, and they either didn't give you the remote for it, or they removed the brain but left everything else there. I'd be more worried about getting stranded some day if the alarm is still in-tact and you don't have the keyfob for it. A dead battery is all it needs to reset itself and arm on its own once you jumpstart or recharge the battery, and if it has ignition kill, you won't be able to go anywhere until you disarm that alarm!