My number 1 garage built in the early seventies shows all those characteristics. I don't think dpc's were standard for garages then. The floor gets damp in winter but layers of floor paint and carpet tiles hold it back. The number two garage which was mostly built by me in 1988 is dry as a bone. It has a visqueen membrane under the pit and not a whiff of damp.
But the dehumidifier keeps the condensation at bay after a quick temperature rise in winter.
