Could be a station assigned PM that follows / responds with a regular ambulance to upgrade them to a Medic unit. It's also popping up in Montgomery County, though it's commonplace on the Eastern Shore. I'm not convinced it's a good thing, but the theory is that if the PM is not needed, they can return to service faster than if a Medic unit was forced to transport a BLS patient. EMS officers have upgraded skills that allow them to do advanced procedures that other PMs are not certified to perform, and generally have years of experience and expertise.