FWIW, I have the Fuel Cell version of the Clarity and it, too, has the same opening and radiator/ filter looking thing at the back. Shortly after I got the car I bumped into a high curb while parking diagonally and upon checking for damage noticed the opening. Concerned that I must have dislodged the opening's grille, I phoned the dealer, and the agent ran out to look at their other Clarities. The dealer was as surprised as I was that there is nothing covering that opening.
2017 Honda Clarity Fuel Cell, Black Crystal
2018 Chevrolet Volt Premier, Green Mist
2010 Toyota Prius II, Pearl White (currently in New York with my daughter for college)