I've been trying to mentally picture how this could happen, and could only come up with something more solid and stationary than road debris acting on the plastic there. The type of damage definitely suggests abrasion, unless as Unobtanium suggests it is rodents (pretty basic rat, if so!). It is definitely odd if there are other parts at roughly the same elevation on that side that aren't similarly damaged. Maybe try to think about where you might be in a situation where the other object causing the scraping is projecting further out, so that the angle could mean only contact with that part of the car as you move past/around it. In previous vehicles, I've backed out of end parking spots where there is a sidewalk or planting area with a curved or angled curb and I scraped just part of my underside as I angled the car out of the spot. This type of abrasion damage could have potentially occurred with a single event, so it is possible that it happened without even your awareness, so no reason to remember such a situation. Does anyone else drive the car beside you? Do you get washed at a car wash where someone else moves your vehicle? Parked by valets at all? Stuff like this drives me crazy trying to figure out, but sometimes there's a really simple explanation but no way to absolutely determine what happened.
If it drives you too crazy (it would me), you could consider replacing that part. While it is unfortunately almost the full wheelhouse trim piece that needs to be replaced (the corresponding back piece is a smaller trim), the part isn't terribly expensive (<$75) nor does it appear difficult to replace. Just a quick Google shows that the part # may be KB7W-51-W20E, but please double check that before ordering (verify correct year, and correct for the front driver's side).