I agree that Huck lied, but is there a source proving that this was what was done? It makes sense to me. My eyesight isn’t that great, but I don’t even see a bookshelf. At first, I thought it was an enhanced convenient “sparkle” from a light on the Christmas tree. Then I thought it was a window pane that was used disingenuously, but it can’t be because the Christmas tree appears to be in front of the cross.
It really wouldn’t matter to me at all because special effects are used all the time, but when Huck said it wasn’t a cross, he blew it. He would have been much wiser to say something like, “Cool how it showed up, huh?” He didn’t. Instead, he chose to lie.
If nobody noticed the shelf beforehand, then why is it empty?
Where are the books, family photos and nick-nacks?
If it was "just a shelf that nobody noticed," then it would be full of these things. Not empty except for three Christmas balls.