You're telling us it isn't, OK, so what is it then?
The OES number for it is 5249844 and the mounting bracket: 5230528.
(I'm in the windscreen business and this information was emailed to me by Pilkington).
Pilkington is right, the numbers 5249844 and 5230528 are the bracket and GPS receiver/module. In post #33 you can see that is definitely the GPS receiver/module. I certainly hope that Ford's ECAT software shows the right parts and part numbers.
The link below shows a rain/lightsensor for a Fiesta without active city stop, as you can see it is totally different. Even the part numbers don't match.
The sensor with active city stop is small and round, but I couldn't find the right picture.
The TS could disconnect the box. When it is removed there should be a message on the radio screen that says 'NO GPS'.