Here is a pic. It’s a 2016 Mazda 3. It looks like a tiny door, except as far as I can tell it doesn’t open. Or maybe it’s something to be punched out and replaced with … what? Here’s another pic of a different model year which has the thing on the other side of the front grille. My sister just had her 2006 Prius’s front bumper replaced after a collision, and the replacement has a similar thing. She didn’t notice whether it existed on the original bumper.
There is not a corresponding hole on the other side of the bumper, so it’s not as if it’s filling in a space where fog lights or some other optional equipment might go.