What happens to special cars

I went to Lago Zancudo, there is a gas station where I fortunately found a tanker (what trucks use to carry gas). I parked the security van next to it and I shot the tanker from a distance. Four shots later it blew up and open the backdoor, dropping a brief case with $7,000!!!!

If you park a car in the garage, it will be saved there. Your standard car will always be available nearby if it is not in the garage. Parking in the garage will save that car and you can access there unless you left it somewhere else.

Taken from the manual:

Player characters can own any vehicle in [GTA V] as long they have appropriate storage for it. To purchase a new car they must first own a garage ... If a vehicle is destroyed it will reappear at any residential property that the character owns with a garage (note that this does not include safe houses). If an owned vehicle is lost, stolen or abandoned, it will be impounded. To retrieve impounded vehicles the character must pay a find of $250 and collect the vehicle