The garage with no number where you had to bring the Securicar to in the mission "Van Heist." is the special 'Securicar Garage'. When you bring a Securicar here you'll get a little money. The first time you bring in a Securicar, you get a $5,000 reward. The next Securicar nets you $4,500, and so on. Thus, you can score a total of $27,500 by delivering 10 Securicars to the garage.
Completing the Import/Export garage nearby this garage will allow quick access to Securicars nearby.

Bring cars to this garage (located in Portland Docks) to unlock a special treat. Once you've completed the list of cars that adorns the wall outside, you'll be able to walk inside and choose any vehicle from the list to drive away in!

The requested cars are: Securicar, Moonbeam, Coach, Flatbed, Linerunner, Trashmaster, Patriot, Mr Whoopee, Blista, Mule, Yankee, Bobcat, Dodo, Bus, Rumpo and Pony.