Returns out of hour at the Meridien hotel are forbidden. You will be charged with a 50 euros penalty if you do so.
The Sixt vehicle can be returned free of charge at the Marriott hotel, Rue du Port in Cap D'Ail (06320). Follow directions for Fontvieille. Head for the Louis II stadium then follow directions for Cap D'Ail and the Marriott Hotel. When entering Cap d'Ail make a left turn to "La Rue du Port". Parking entry has a "Parking Privé Marriott" sign. Park the Sixt vehicle on a Sixt space and leave the key inside the Sixt key box located on the bottom left of the automatic cashdesk.
Your way to Sixt:
Enter Monaco and then follow directions to Larvotto. Once reached the seaside, head East. The Sixt branch is located inside the Le Meridien Beach Plaza Hotel.
Please stop in front of the hotel, Sixt staff will assist you.