I, too, had a couple of very tiny marks that came with my Air. This is the first device of 10 Apple devices that my husband and I have purchased over the last 6 years that came with visual defects albeit small. I considered bringing it back but bought it at Best Buy and just know that their return policy is not all that customer friendly. My Air also has no screen issues as a lot others were complaining of. I ended up with a white carbon fiber type skin on the back so now at least I don't look at the marks.
I exchanged my first iPad Air due to yellowing on the left side of the screen. Got my replacement and immediately noticed a chip in the chamfered edge near the top of the screen. Not terribly bad but it's become the first thing I look for in a new or replacement device after going several rounds with scratched up iPhone 5. Ultimately though the screen on this unit was night and day better than the first iPad Air I had. Given all the things I had read about people getting screens with yellowing or uneven backlighting, I decided I could easily live with one small nick rather than risk getting another replacement with a worse screen.
If it really bothers you, by all means return it. But I would think twice about it if your unit has a good screen that you are happy with. There are definitely some less-than-great iPad Air screens out there.