Billy-I could not agree with you more. If the spa is located in even a semi-residential area, and neighbors begin to see an assortment of unknown autos, trucks, and people in the area, it can become an issue.
Rear Parking Lots in a small or strip mall can be very good. For example, in the case of L.H. the lot in the back was perfect during the heydays of that place when they had a steady flow of visitors. You could safely park in the rear lot, walk up to the rear entrance, knock on the door, and typically be greeted by the friendly Mamasan.