Many fleet cars, even outside of Silvercar, do not have a SunPass transponder; they instead use a process called PlatePass which captures the plate and transfers the toll back to the company registered for the fleet.
That's partially because regular fleets have nationwide one-ways, unlike Silvercar. The first time I heard of PlatePass, we had a fully-loaded Hyundai Accent with VA plates from Avis in MLB. There was what looked like an oil change sticker on the front upper left corner of the windshield, but instead it explained PlatePass.