My wife happened to see a programme on British T.V about a year ago, about 2 French pioneers who fit your description and who disappeared on their return to France from showing their moving pictures in Britain. From what i can gather, unfortunately the mystery wasn't solved,but possible explanations included their being murdered.They were thought to be gay, and were last seen with some unsavoury,possibly homophobic character(s) apparently.They were trying to advertise their work back in Paris,which also may have led someone to want to steal the credit,have them "done in". I'm sorry i can't be clearer:this is second-hand info,but it does appear to be the 2 men in question.The programme concluded that the reason for their disappearance would probably never be known for sure!