There's a site that shows all the tag locations (Found here) and there's one final one I can't find. It's in the Brotherhood territory at the southern point of the map in an airport. How do I find it? Anybody got any way to get to the point, all I really know is that I think it's underground.

The map isn't working for me right now.  I know where two are (not sure how many are at the airport).  The first one is on the outside of the front of the airport terminal entrance.  Facing the entrance, just head to the right (north west) along the wall until you see it.  The second I know of is also outside.  If you look at the map, at the southern-most end, there are four black rectangles.  The tag is just south of the bottom right (south-east) black rectangle on the map.

Hope that helps, if you haven't already found it by now.