too much moisture in concrete slab
what you might consider doing rather than searching for a mysterious leak is to install a raised floor on top of the concrete. This is simply a frame work of treated lumber fabricated on the concrete. To solve the moisture issue a sheet of plastic is installed between the treated lumber and the old concrete floor. If you are installing a sheet floor covering you will need to cover the treated lumber with tongue and grooved plywood. This suggestion will only work if moisture is minimal. If you have standing water the source of the leak must be found and repaired. This solution is for a floor that is damp, but not wet.