You took those words out of my mouth. I thought exactly the same. Cracks tend to go off in a straighter line and deviate in a more uniform manner.That looks more like a scratch rather than a crack.
Looks like the phone has possibly come up against some keys or other hard object in a pocket or bag.
If it's of any consolation to the OP, my G4 Cube had and still has stress-relief cracks in the casing which was a known issue, which Apple never found a fix for. Add's to it's vintage charm........
As it's a brand new phone it will be interesting to hear back if Apple change this under warranty!