How is my phone already broken after a week from purchase?

[AC Question]

I have recently recieve a samsung galaxy s7 edge about a week ago from my parents. Immediately after I got it I placed it in a otterbox case that's comes with a clip on cover. I've had it on constantly even when I'm not using my phone I clip my phone to the case so the screen would not get damage by any possible means. I have yet to drop it. But somehow in some way, it got damaged. From the screen, if you can reflect it right, on top looks like an oil smudge when really it's something underneath the glass that I can't touch. It's only visible when I look at my phone at it from an angle, but face front the screen is fine and I can still touch the area. However, I would like to get this removed but I would like to know how it got there in the first place when I always kept my phone in the case and never in a tight space where pressure can be applied to the screen.

 

[Almeuit]

You say you got it from your parents -- did they use it before you?