This was a lot easier to identify. It's an antenna.
It's a "Fractus Slim Reach Xtend™ Bluetooth Wireless Headset Antenna", designed to also support 802.11b/g/n (basically anything in the 2.4Ghz range). It is tiny, only 7x3x1mm. It's not a particularly efficient antenna (rated at >50% radiation efficiency), but it was probably chosen for size. It's still a good one, I can be on the side of the house opposite of my phone and still have good reception.
For only 1mm thicker, Bluetrek could have used the more efficient "Compact Reach Xtend™", with a radiation efficiency rating of >70%.