We have just come back from Menorca, and in two weeks we only saw 1 white ST (a taxi). I know its only a small island but was quite suprised with it being a Spanish Island. Lots of Ibiza's (probably rental cars).
I returned from Ibiza last month and thought the same thing - only saw one or two. Saw lots of Fiat Panda rental cars, and apparently the Spanish don't really buy a lot of Fiats, so maybe holiday islands dont show a true picture? But I did expect to see more Exeos than I did!