This is somewhat of a follow up to my mystery post.
Yesterday was the first time I was looking for a parking space, and the first available spot was next to another Model S! On this occasion, I was at a location I have never been to before in any car. The event had no relationship to any official or non-official Tesla gathering.
It took less than two months for me to be confused by another Model S in the same parking lot, and less than five months before I parked next to another Model S. In four years of driving the Roadster, only on one occasion did I see another Roadster in the same parking lot.