Circle on the back glass. Cracked?

[Bioback]

It's been two weeks since I bought my Z2 to a local carrier and a few days back I noticed a little circle on the back glass it's really tiny and can be seen only against light but anyway I wanted to know if it is suposed to be there or if it is a crack or something.

Thanks in advice.

[Thommo]

If it feels smooth then it could just be a slight defect ie: An air bubble when the back glass was made

[dm841]

did it drop by any chance 

[Bioback]

Yes it does feels smooth
No, I'm absolutely sure it never fell