Do you carry your car keys with you on the trail, or you do hide them at the car? There is no point to carrying extra weight, and car keys certainly fall into that category. I leave all of my keys in the car EXCEPT the car key itself. That goes in my pack.
Or do you really practice ultralight backpacking, and leave the car key artfully hidden somewhere so that you don’t have to tote the extra weight? (And no, please don’t tell me or anyone else WHERE you hide those keys…)
Of course, true ultra-lighters will either take public transportation to the trailhead…or buy a car with keyless entry!