iFixit

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%.

Here is the (mostly) disassembled Bluetrek Skin headset.

The Skin is a lie!

Everything just snaps back together.