I can confirm it not being a problem. I have taken a few unopened phones through T5 security (old fast track, north fast track and First Wing security) and not had any problems at all.

I also took a brand new iPad through T5 security. They wanted it in a separate tray but they did not want the box to be opened. Thank goodness, as this one, along with two of the phones, were gifts, and I did not want to open them if I could help it.

It makes sense when I think about it. If they can't see through a paper/thin plastic box with the X-ray, they can't really function as a proper X-ray screening system