The butterfly and the rose, hidden in the beginning credits (on the roof, and on the windows of the bar) are references to the tattoos of Mathieu Vié's girl.
The number 9 on the door in the beginning credits is a reference to Mary's address.
The name of the hotel in the beginning credits (NEFELI) is the hotel where Mathieu Vié was staying in Athens when he met Mary.
The bridge represented in the beginning credits is Sainte-Marie Bridge.
There's fourteen butterflies in the end credits, relative to january the 14th. This date will stay secret.