Out of hours return
A keybox is available in the Indigo Parking lot located at 2, avenue du Coubertin, 92100 Boulogne Billancourt.
Enter the car park, take a ticket that you leave on the dashboard, go to level -2.
Then you will see the SIXT places at the bottom of level -2. The keybox in which you can leave your key is between places 54 and 55.