Lost: Backpack with personal items

Left estepona today, and with a mix up with our transfer to the airport, my backpack was left in a drivers car. We got into a black Chrysler car, European male driver(Swiss or Dutch) very friendly, who was picking up travellers from the same pick up point, Madrid apartments on av. espana. Not sure if he is a registered taxi, said he was doing an airport run as a favour for a friend who rented out an apartment in same building.  Would really like to track down my rucksack. I will be moving to estepona in a few months, so can pick it up from a central point. Would appreciate any help to track it down, perhaps you know the car/apartment owner, all help/advice welcome. Thank you