I had my key fob in my hand when loading a big box in rear hatchback. Got in car and drove away. Got home and tried locking car with car handle sensor and got the long tone and message on dash letting me know key inside the vehicle. I've gone through that car and can't locate it. I can get in the car and drive away, but so can anyone else. Since there is a sensor on the fob that tells the car the fob is inside, is there something that will allow me to locate the fob. Or does someone know of a spot it could have fallen into? When loading the box, I had opened both rear doors and the hatch. Took everything out of the hatch including the spare tire. Removed everything loose out of the car so it is in the interior from backseat rearward. Checked front seats just to be sure. Now I'm asking for help.

I think you need to find out how to remove the bottom portion of the back seat.
It probably went down the crack between the back seat and back of the back seat.

You were right. Fell down and went under the rear seat. Went to dealership and they removed the seat and got it for me free of charge.