Matt, I find it very hard to belive that water is doing what your speculating. That would be a maze to travel in between the mastic and the grout and the obstacles it has to manuver through to come up where it is.
Nothing on the ceiling, nothing on the walls, and nothing in the basement???? This is very strange. I cannot even speculate on this, but if your set on your theory, then I would hit that exterior wall with a caulking gun and start caulking with a good latex/silicon compound and experiment and over many heavy rain falls, you might just find the entry spot.
Otherwise, I would do a "Jay" and document the situation with pictures and documentation. Put a quarter by the water spots to show size and keep this for your records. When this happens, perhaps show your builder if still around.
YOu know, this will have to get worse in the future and begin to show damage somewhere, maybe where you do not see. It is like a small cavity in your tooth that does not yet hurt. It will and it will have to be addressed.