First of all, notice the direction of the light and shadow.
Secondly, The Queen steps into the vault and there is no shadow of her from light behind her cast onto the vault an the first picture shows the light was not coming from behind her.
So , knowing this, it doesn’t seem this reflection was just an accident in lighting, some genius on set probably got the idea to highlight what would be Grahams box. Could just be a shout out to the people who are looking. Also, it seems too highlighted in comparison to other reflective surfaces present to just be a mistake.
[adrotate group="5"]