Had my bumper off the other day and noticed that there was a bunch of gunk accumulated on the inside of one side of the front bumper (but not the other). Turns out the access cover was missing on that side and letting the gunk in. Looks like replacements are available pretty cheap:
https://www.ecstuning.com/b-genuine-mini-parts/lid-wheelhouse-cove/51719802413/
https://www.ecstuning.com/b-genuine-mini-parts/lid-wheelhouse-cove/51719802413/