In case some here don't know, its only available once you clear a set of cases for Traffic, Homicide, and Vice. In the beginning of the game when you at the silloutete menu option, keep scrolling until you get to the CASES option. Once there, select the type of cases you cleared and Free Roam will be the last option. Did it briefly for the Traffic Desk cases mainly to finish up the street crimes. I also saw some of the secret cars while taking care of the street crimes. Some of the designs are interesting to say the least. Its a shame the secret cars don't have police radios and therefore can't be used during cases. I don't think you can save the cars in any sort of garage for future use at all.