Ewarmour,
My office is in an industrial park in Santee between the 67 and Magnolia. There are 2 huge pits located there and the land is owned by RPG Brick. If you ask them they will let you fish there. I have been fishing those pits for years and have never had a problem, ask and ye shall receive. The posted signs are generally, as mentioned prior, by property owners who don't want people cutting holes in their fence.