One thing I don't get...

1. I don't think Dobbs was "sure" but it was through his investigation and deduction. He learned from Mélancolie's mother that they saw and talked about the passenger getting off the bus. He learned from Nicole who worked at the shop that the guy was peeking through the glass at Mélancolie when she was in the changing room. Dobbs also knew that in that area no one could get around without driving but the guy took the bus and so had no car. So he deduced that he was hiding in the back of her car when she drove home. After that he disappeared into thin air, so where could he be? The passenger was a notorious serial rapist known to like to stay around in the victims' homes afterwards. Since he disappeared totally Dobbs thought the victim had killed him. He also found out from the cop friend of Mélancolie's husband that she drove out at night and told the cop that she was driving to the airport to meet her husband but it was found out that she didn't go there. That led to his suspicion that she was getting rid of the body. So he started investigation in that direction and later he was shown to be correct.

2. The wedding refers to the wedding of some woman that Mélancolie attended at the beginning of the film and we saw Dobbs for the first time looking at her. I think she looked nervous and he knew something was up.