See with the Lost&Found at St Pancras station if you lose it in Eurostar. If it was in Paris, it's here (sorry it's all in French, ask me if you need a translation) http://www.prefecture-police-paris.i...ts_trouves.htm
It's kept for 1 year but you have to go there to recognize it and get it back.
But ... I'm afraid it was may be stolen unfortunately!