Check the outside area for excessive moisture (detached/clogged gutter, bad grade, etc). If your seeing efflouresence there's a good chance water is passing through the foundation in addition to any that is condensing on the surface.

Any chance this is on the North side of the house or in an area with little or no sun?

Also, check for any other sources of moisture for the garage. Bath fans, laundry equipment, etc. The furnace and water heater could contribute too, so you're on the right track.