I agree with reem1 that Mark is too keen to be involved.
He doesn't speak any French, which would seem to be a basic requirement for a police officer liaising between French police and an English speaking couple. Luckily for Mark, Baptiste, Laurence, and Vincent all speak English fluently and kindly translate for him whenever he doesn't know what is going on.
Mark doesn't seem to get on brilliantly with Tony or Emily either. He helps Baptiste track down the information about Tony's assault of Greg. Tony gives Mark a really dirty look when Baptiste says that he wouldn't have been able to track it down without Mark's kind help. In the same episode Emily shouts at Mark to "Do your bloody job." when Mark is being questioned and he won't tell her what is going on. At another point Mark tells Tony and Emily that there is a Paris lead, but then won't give them any more information.
Is it significant that Mark is one of the few people who know that Baptiste and the undercover policeman are going to meet up to discuss things after the failed raid on the warehouse? Undercover policeman is, of course, murdered just before the meeting.