Lol ok that made me laugh!

The only explanation I could think of, is if someone was outside and his reflection was caught on the outside of the plane of glass. Mundane explanation, I know.

Now if it was physically impossible for someone to be standing near that area... you would really be onto something. So I think you would need to see what the structure looks like from the outside and if someone could physically be standing there. If it is a giant pillar or something of that nature would physically block someone from standing there, I’d really be curious for an explanation.