i would imagine if your garage is sealed from above and there is no water getting in from anywere you are refering to surface run off water. I had the same problem with my garage. I phoned a local builder who sealing the edges of my garage with a small concrete border and i have had no problem since. It does get damp in there sometimes but not as much as it did before. All I can think of its condenstation build up in the garage.