Remember just because even if the attendant does not notice/mark off the damage it may not be the end of things. All rentals are subject to audit which can occur days or weeks after you return a vehicle. No one may notice the damage when you return, but the next renter of that vehicle or the hiker transporting it may and have it noted so it isn't charged to them. If when that happens depending upon things on the ground you could see actions from National on your credit card.

Basically you have two options: return, point out the damage and see where things go from there; or return and take your chances.