My dog seems to get sketched out when I make faces at it in the mirror. It seems like he doesn't really know what to make of it, but uses it as predictive power

My dog seems to get sketched out when I make faces at it in the mirror. It seems like he doesn't really know what to make of it, but uses it as predictive power none-the-less (he's seen me reach for a leash in a window reflection and got excited). I think they know the image is on a 2D surface though, because they don't react to dogs on TV either.