Strange backlight issue on iPhone 5

I pre-ordered two iPhone 5's and received them on launch day. Both are 16gb, one is white and one is black. My wife's phone (white) is having a strange issues that I can't figure out.

She will be using her phone and without notice, the backlight on the screen will dim ALL the way down, you can barley make out the screen. Pressing the home button will restore the screen brightness, as will locking the phone. I have checked the screen settings and the brightness is set to 3/4 and auto brightness is OFF. I have also done a a reboot on the phone. The problem appears to be random. I can't figure out why it is doing this and of course, she is wanting me to fix it. The strange part is that it will occur when the phone is being used. Any input appreciated!

 

Thats exactly what I thought when she first told me but she says she isn't. The phone did do this for me as well and the light sensor wasn't being covered. Not sure if this is a hardware issue or some kind of quirk in IOS 6.