iphone 4s crack under the glass

I recently noticed a curved crack/scratch on the top left corner of the display and I wonder if anyone has had the same issue and if this is covered under warranty ?

This is located under the glass, the top surface is perfectly fine.

When I directly look at it looks like a crack but looking at it from the side it looks as if the adhesive film had come off. I thought it was the lcd but no lights were bleeding through when looking at it under a blanket.

It's just strange as I've never dropped this phone before and there is only a lcd screen directly under the glass ? I wonder what could damage the glass from underneath.