Fortunately, you had the keys with you a moment ago and still haven't fully ventured inside, so they can't have gone far. You search the ground around you and around the car, kick through the nearby grass a little, but find nothing. You finally commmit to the search and get down on your hands and knees to look under the vehicle.
While you don't immediately see what you were looking for, it does seem as though a modest sinkhole is starting to open beneath your car. Looking over the edge, you can see a tiny glint at the bottom which could be the traces of light playing on your keys, but it's impossible to know from way up here.