identifying drainage problems without pulling up the floor
My initial reaction is that it is still moisture from the January incident.  

Concrete can be water-tight but most poured slabs are still very porous and water will seep into many of tiny bubbles of the concrete and persist for a very long time.  Though the remediation company may have come and dried the floor, the slab may still have water and it is evaporating.  Replacing the floor too soon may have just trapped the moisture, slowing its drying and placing your flooring into nearly direct contact with the moisture, barrier or not.  Your climate could affect this too, live in the southern US for example it is going to take a lot longer to evaporate than in Colorado.

I personally would rip up the flooring, buy just a consumer grade dehumidifier and run it for a few months; wait a month without it running and then test for moisture again.  Testing the sheetrock near the floor with a hydrometer will also help determine if there is a moisture problem too.

It is a pain, but the pain and patience will protect your investment.  Selling a house with suspected mold is not always easy, but not disclosing the damage is also risky.

By drainage problem you are a bit vague... or they were.  If it is a problem with the sanitary line, it might require a plumber to determine that and at worst, digging up the slab!  Drainage problem could be also that your landscaping is holding water up against the house which eventually could make its way into your living spaces as moisture.  



One other thought.  

If the flooring had been reinstalled in the winter, it could be the changes in humidity that could be stressing your flooring. But given your situation and what some professionals have told you, I would find it unlikely.