As much as you say it hasn't been hit, that looks like a classic impact mark.
Something has pushed on that portion of the bumper and caused it to push in and touch the bolts behind.
Do you know when you first noticed this? I'm currently in a legal battle with a dealership regarding pre-purchase damage on a 'new' car (the car has been rear-ended on the rear left side prior to our purchase, yet that wasn't disclosed to us)... this might be a similar story if it's been there since purchase.
Previous Rides: Polo GTI, Mx5 10AE, MY05 WRX WRP10, Renault Sport Clio 172
Current Ride: Evo 8 MR, Fabia MK3
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