What could cause a wet soil / moisture smell on concrete slab
It sounds like moisture under the slab.
Have you tapped a pice of plastic down to the slab for 24 hours? Taping a trash bad down then remove it the next day if the bag has water droplets on it or the area is wet under the bag the slab has moisture under it that is weeping up through the concrete. I have good luck sealing these floors in summer when the water table and possibly a spring are not causing the problem, in winter the moisture coming through will prevent the epoxy coatings I have used from sealing. I have fixed several homes with this problem by acid etching the slab and applying 2 part epoxy, it has worked well for me and may solve your problem.