The Castle Pinckney flag is probably the "Fort Walker" flag at the SC Historical Society. It originally flew at Castle Pinckney before being moved to Fort Walker, but I cannot remember if it was flying at the time of the firing on the Star of the West.
We tend to forget that they did have binoculars in those days (strong enough to see signal flags). And Foster definitely was watching the surrounding batteries/forts closely. I was pretty amazed when reading the ORs, the first time, just how much he reported up to Washington.