Somehow this vent came loose on my STi. I know the person who did my clear bra removed these vents, it is possible they did not put it back properly. I noticed it was loose while giving it a wash this weekend.
It was not snapped in properly and I could not snap it back in, so I searched and surprisingly found this very helpful thread! Thanks.
I was able to undo those three clips in the wheel well (and also one on the bottom of the bumper), then just reach in and snap the vent in from the inside.
Unfortunately one of the three clips that snap in the actual vent is missing, but with the top and bottom one secure I'll wait until I am at the dealer next time to pick up another one for the middle. It seems the middle clip is largely redundant because there is very minimal, if any, play in the vent when using only two of the three.