Jason,
Before you get all dirty going under your house, check the siding to ascertain that it is not allowing the rain to get in. Second, make sure the plumbing is not leaking under the sinks. You sometimes "see" a leak. Get a light under there and feel each line including the traps with the water running. If you don't find it, get back to me with more info.
Len

Hi Jasonrkr,
You might also check your water meter before going to bed...take the reading and check in the morning before you run water.See if the reading is the same.

Iceman and Matches,

Thank you both for taking the time to respond. I really appreciate it.

I have checked and don't see any leak and I also don't see the meter moving. My house is on slab and I have stucco exterior. Any other suggestions?

Thanks,

Jason

Hi again jasonrkr,
Have you had alot of rain recently?Are you at the bottom of a slope?Any air conditioning units in the house?If you aren't losing water we can assume that this is either happening when you run water,have rain,or experience condensation of some sort.That's about as far as I can go at this point.

Dear Jason,
Another thought, check under the cabinets while the water is running.( Both hot and cold). It may be a trap leaking or it may be a defective faucet. Remember, the more info you put in your question, the better I can assimilate where to look. Get back to me and if i can offer a solution I will.
Good Luck, Len
P.S. Fill the sinks to capacity and drain them. Sometimes a leak won't show up unless one goes to extremes.

Dear Jason, I've pondered your problem and had another thought. Check the outside of your home to ascertain that your stucco is not in contact with the soil. If it is, you could be a victim of "Whicking". Stucco is porus and will whick up surface moisture. To understand this, place water in a pan, hold a paper towel to the water at the extreme bottom. watch what happens. This is whicking. While the moisture is waiting to evaporate, mold will form.
Let me know, Len

Iceman,

You're great. I have not solved the problem yes, but the suggestions you gave really made sense. One more question if I can. Do you know about a foam insulation? My wife mentioned it and I was wondering if I can "do it myself" or if you need to be a contractor?

Dear Jason,
If you're referring to the expanding foam insulation,yes, get a contractor. By the time you rent the equipment, buy the safety gear required, go through the trial and error method I did, you'll have spent twice as much money and lost most of your hair in the process. If you're referring to the sheet type insulation that goes under aluminum or vinyl siding, that's a diyp.
It also takes alot of prep work over stucco. I.E. firring strips, caulk, etc. etc.
Good Luck & Merry Xmas,
Len