It also reflects the IR that is around the camera (usually) This tends to give a bullseye effect where there is a brighter ring in the centre of view. You can see it in the picture so whatever is there is reflecting the IR light.
But you can also see through the skirt part to the horizontal block behind it. If this was a dress then it would block the IR light, but IR has a strange ability to pass through low density polythene - which is why they make the PIR lenses out of it.
The apparition was caught on camera at the site in Sherborne Street, early on Tuesday, August 18.
www.birminghammail.co.uk
Here is the story in the local "Birmingham Mail" newspaper. One comment at the end of the article offers this explanation:
"I work on this job and this is not a ghost it is 2 lady's walking next to each other one in a white dress and one in a red dress i have seen the security camera recording"
It is interesting the only image we've seen is a still lifted from the video. Has the video been released? If not, we may know why if the commenter is correct.