This sounds like moisture to me. I have had Traffic in my personal home turn a milky color due to high humidity airflow (near a door that was not airtight). Based upon your measurements of the spots, I would want to know if you have duct work or something else directly under the spots. And what moisture readings of the floor were. The milky spot on my floor went back to normal after the humidity was corrected. If no moisture sources are present underneath, I would tend to agree about solvent being trapped.
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