With the plethora of surface lots in downtown, Ive often pondered the same thing myself. My assumption has always been that, knowing Houstons history of demolishing historic structures, several of those lots probably used to be old circa 20's or 30's buildings. Im pretty sure there has to be some sort of map or grid of downtown that shows what old buildings used to be up on what lots, maybe even with pictures. If anyone knew where to find out it would probably be a poster/s on this site.