If there's no gap between the cover and sealant somebody screwed up when it was built. It' a vent for the roof between the outer roof and inner ceiling. It vents moisture & heat from that space. The plastic gets very brittle and they break easily. I have removed both my plastic vents and installed steel vents. The steel vents need some fine mesh added when you install them to keep the bugs/rodents out.
2012 Sanibel 3500
11 GMC 3500 dually, Duramax/auto