This is a great story. It has all the makings for a study is psychology. How many people would notice when the valet pulls up and hands you the keys to the strange car you only spent a half hour in two days ago when you drove in from the airport? How big would the difference have to be before everyone noticed? Black Chevy vs. white Ford? If the man at the valet stand asked you what kind of car they were picking up for you, would you know? Generally, I think I would. but, if they brought out a similar color car to me I not so sure if I would notice. I recently had a rental car (while my car was in the body shop) and it was a Chevy 4-door, but there was no model name on it. I still don't know if it was a Malibu or a Cavallier or what. I had great fun with it asking people what it was. "It's a Chevy." "Yeah, a Chevy what?" They would then walk around the car looking for a clue, but there wasn't any.
My other thought is that if the other couple waited 15-20 mins. for "their" car, maybe they just didn't care to bother making the correction and waiting another 15-20 mins. Maybe they were already late for their tee time. They probably had a running joke for the rest of the week as they got to valet, "Let's see what car they bring us today dear." (They're probably the same people who break all the rules too!)
This reminds me of an episode in my former life as a police officer. The department I worked for issued a car (cruiser) to each officer. We discovered over the years that the key combinations are not as infinite as you might think. When the department bought 200 Ford Crown Victorias, there were probably only 20 different key combinations, so your key would work in several other cruisers, you just never knew which ones. One night there was a call and an officer ran outside the station and unlocked what he thought was his cruiser, got in, started the engine, put the car in drive and it was at this point that he felt hot breath in his right ear and heard a low rumbling grrrrrrrrrr.

The car that he got in to belonged to a K-9 officer and there was now a 110 lb. German Shepard standing up in the back seat very curious as to who was trying to drive him away. I should add that there was no cage separating the front and back seats. The officer then slowwwlly turned the car off and eased out of the front seat while sweetly saying "Good boy, good boy," He then got out and quickly shut the driver's door as the shepard slammed against the window going nuts. Needless to say, the officer took about 5 mins. to catch his breath before going on the call.