I think it's the Hawaii requirement for registration sticker display. They're on every single other car you see here, not just the rental cars. Basically 2/20 means the registration expires on that month/year. Makes it SUPER easy to know which car is newer vs older. They're all on 2 year cycles, so if you see a 7/20 means that's a very new car (registered in 07/2018) vs a car with a sticker that ends in 19.