If we look at some of the other photos taken during day light it is obvious HM was not wearing a brooch in her hat.
Yet, because of the angle and because of the chiffon on the hat, at other times it appears that there might be a brooch on the hat.
No brooch visible
Definitely no brooch
An optical illusion could lead to the suggestion that HM was wearing a brooch on the other side of her hat in this image.
The angle from which photo is taken highlights how at night time the hat could lead to the impression that something was pinned to it.
In the photo Brock posted one section of the chiffon has caught the light but this section does not have the look of a brooch to me.
Below is a link to another image from the same evening. To me it suggests that the material in the hat has caught the light as it comes out from the join. http://www.paimages.co.uk/image-details/2.5883729