I had a home (1999) a few days ago that had an corner of the concrete in the garage that was "sweating" according to the homeowner. The area was dry when I was there, but obvious efflorescence and appeared to have been wet previously. This area is in the same corner as the water heater and furnace. When the garage door was down I tested the water heater for backdraft, but nothing. The furnace is a power vented system. Any ideas?