If the flex magnet or tape suggestions don't work from inside trunk/quarter, then try a Rare Earth magnet, which are strong. Place a microfiber rag over the magnet and place the magnet on the class A surface (outside quarter panel/rocker) where you believe it is. Once you feel/hear wrench drag it toward rear quarter (trunk area).

I used this type of magnet/process to assist operators locate torque converter nuts when dropped into trans housing during assembly (trans to fly wheel secure). There was no access hole at the bottom of trans, so the operators would place the magnet at the bottom of case then they dragged the magnet toward starter pocket where they were able to grab lost torque nut.