Once you recive a message about a hidden car and you press
during the notice, you will see a purple circle on your map, thats the area where a garage with a torn down car will be. You need to drive up to it to let your mechanic repair it.
This site helped me achive this one and the rest of them.
https://ar12gaming.com/articles/forza-horizon-3-barn-findsUsing your Drone makes it easier to find the garage.
Press Meny button and select Drone Mode