numStructOffWorld([player[, structure type[, looking player]]])
Returns an array of structure objects in your base when on an off-world mission, NULL otherwise. If no parameters given, it will return all of the structures for the current player. The second parameter can be either a string with the name of the structure type as defined in "structures.txt", .stattype. The third parameter can be used to filter by visibility, the default is not to filter.