If you are referring to the paved area between Rolph Nicol Park and the Lowell football field, that is part of Parking Lot 25 of S.F. State University, with a street address of 498 Winston Drive. This is shown on a campus map:
https://parking.sfsu.edu/sites/default/files/maps/614_color_map_PARKING.pdfExtensive Googling indicates that the rear portion (north portion) of San Francisco State's Lot 25 was once the site of an Enterprise Rent-a-Car location, with a prefab trailer serving as the office. This would explain the presence today of old concrete foundations. Some out-of-date internet listings of car-rental businesses in San Francisco do, indeed, show an Enterprise location at 498 Winston Drive. Apparently that location was permanently closed many years ago.