Since purchasing my 2014 in May I've noticed something like that in my dimming rearview mirror, and it's very visible at all times. I've just figured it was residue left by something stuck to the mirror by the mirror manufacturer, someone in Toluca during assembly to tell workers what parts had to go in, someone during shipment from Toluca to Southfield, MI, or even someone at the dealer in Southfield.
I haven't gotten around to cleaning it yet since I honestly never think to clean the rearview in any of my cars. It looks vaguely like a barcode, which is what leads me to believe it's just residue.
If it doesn't clean off with Windex or Stoner's Invisible glass, try using a fine mesh netting in conjunction with the cleaner to lightly abrade the surface without scratching it. Invisible Glass cleaning wands available at many auto parts stores come with just such a netting for removing stuck-on junk like that. They're also fabulous for cleaning front and rear windshields without straining to reach, and I've used that netting bonnet to clean rubber transfer from the window weatherstripping. You can also try Goo-Gone.
All else fails, tell the dealer and find out if it can be replaced under warranty, as it may be something beneath the glass.
