Does the bathroom have a vent fan? Also, are you running the vent fan in the bathroom?

A bathroom vent fan is an absolute necessity. And what kind of door is on the closet? Solid or louvered? The closet likely needs some air turnover to prevent the growth of mold.

If you want ammo to prove there's a problem to the contractor, you can get a moisture meter. We bought a professional model (Tramex Moisture Encounter -- we purchased it from inspectortools.com) when we were going through our stucco issues - it was a couple hundred bucks then but I will tell you, that thing has seen a lot of use. We have loaned it out to several friends/family members trying to diagnose their own random leaks and have also used it to track down plumbing leaks in the walls. At the time, you could only get the pro models but now I see that Lowe's carries one for $40. No idea if it's any good but here: http://www.lowes.com/ProductDisplay?partNumber=562134-56005-MMD7NP&langId=-1&storeId=10151&productId=50284821&catalogId=10051&cmRelshp=req&rel=nofollow&cId=PDIO1.  You may find one on Amazon as well -- just check reviews to make sure whatever you get actually works well.

You want the non penetrating kind - you just hold it up to the drywall and it tells you the percent moisture in the walls. It's very easy to find leaks behind the walls with one.

If you have evidence to show there's excess levels of moisture somewhere in the wall, you may have better luck convincing your contractor there's a problem. Good luck!

Maybe get someone in with a moisture meter and test the walls/sheetrock. If an area has moisture you might just want to go ahead and tear out some sheetrock to see if you find the source.

Contractor came by and was shocked at how much moisture was there, seems to be groundwater under the house rather than a leaky pipe. I don't know why I am surprised, I knew that about this house, but I guess when I repeatedly mentioned concerns about water under the house and they thought I was worrying too much, I eventually gave up. Kicking myself for not pushing even more on the idea of installing a French drain around the foundation and adding a layer of moisture barrier between the ground and the insulation when it would have been easy, ugh.
Also, I am now re-concerned that new part of the foundation has no vents installed at all. My contractor suggested ripping up part of the closet floor to dig a gravel pit and install a sump pump, and my father suggested also cutting in vents and installing a fan underneath. I can't believe this is happening 6 months after "finishing" our beautiful reno!!! Frustrated and confused, any suggestions?
Oh, and this is in addition to finding out that the contractor probably incorrectly installed our bathroom fan because water dripped back out of it all winter after two showers, the AC unit that he promised was worth saving has now conked out, and I have to decide if it is worth redoing the roof when they remove the old unit, even though it has about 7 years left in it...and also figure out if we should run an external duct because the AC guy ducted our bedroom improperly and it blows directly on our faces at night and the air doesn't return properly (when it works at all).
It sounds like I am someone who paid no attention to anything they were doing here 6 months ago, but I actually really was and unfortunately found myself having to pick my battles during the whole thing. I don't know who I am more frustrated with, them for not being more diligent or myself for losing track of certain things and not figuring out how to put my foot down or speak up more or something. It is a hard balance to keep a positive working relationship and also keep vigilant about soooo many different aspects of a project, oy. I feel for all of the contractors, architects and designers who coordinate all of these elements routinely, you really have a lot of factors and personalities to manage on these jobs!!!!

You might in a position to have your original contractor circle back and (at least) install the foundation vents that he should have installed in the first place. I would definitely mention it to him. I am not sure what your circumstances are, but around here enclosed foundations require ventilation, which should be installed when setting the blocks.

You might want to post this in the remodeling forum. There are people over there that can help. This is more than a bathroom issue. So sorry you are going through this.

Is that an old pipe or new plumbing that went in with the addition?

New pipe :(