Once you enter the hotel, turn right and you will see Sixt’s reception - check the photo I attach to see where it is located. Lady at the desk was very polite. Many big companies fail to provide personalised service, whereas Sixt proved being one of the exceptions again.
They bring the car just to the same hotel entrance and that’s where you would leave the car - nice bonus! And little tip: try to get smaller car since parking spaces in Monaco are pretty small.