Alright now I understand. Sorry, I was on day 3 of staring at a set sheet for a commercial bid... that's another thread through.
Did the original contractor build the house addition over a live lateral, which is now under the house? I have never had that happen on a house addition, but I have seen a lot of patios built over live irrigation laterals that ended up breaking.
You might call in a plumber or rent a snake with a camera, then scope the drain tile and see where the water is coming from. It will at least get you closer.