The county "discovered" this fleet of no-mileage vehicles after reading about them in a Spanish-language newspaper there (see the source for more images). Most of the misplaced motorcade is made up of Toyota Prius hybrids whose warranties either expired with very few miles on the odo or will very soon.
Looking to save some face, the county has rushed at least 123 of the hybrids into service. The Toyota warranty covered the hybrid bits for eight years or 100,000 miles, but we're not sure if that covers cars parked for five of those eight. We're also not sure what that much time in Miami heat and humidity does to an unused hybrid powertrain, but it can't be good.
The county, as you probably guessed, is looking into how it lost so many cars. The leading theory is that they might be part of Carlos Alvarez's time as mayor. He was the mayor during the period the Toyotas were purchased, but a 2011 recall election successfully removed him from office. Apparently the voters "felt, among other reasons, that he had been behind multiple acts of misappropriation of funds."
We've tried to contact Toyota to see what kind of warranty support these cars will get but haven't had any luck so far. We'll update with their response.