Mystery damp spots on hardwood floors

Friends have purchased a bank-owned house and are remodeling / refurbishing the floors. Pulled up old carpeting and backing in 3 rooms. Backing was deteriorated and stuck to 60-70% of the floor. They are sanding down everything and in 2 or 3 areas, some damp rings appear. 2 other rooms never carpeted have no problems. Although the house at some point had some "pet" problems, there is no apparent current source for the moisture/dampness (no leaking roof vents through the walls, no plumbing anywhere close, no condensation piping, etc, newer roof, attic looks good). House was probably closed up for awhile, but no basement water problems either. Is it possible trapped moisture may be coming out of the floor or sub-floor itself, or something in the carpeting padding, that may either have trapped moisture or even attract moisture on humid days? Actually chiseled up a section of damp appearing wood. The top 1/32 inch seemed slightly damp to the touch, but the bottom 90% part of the chip appeared dry, no discoloration, etc.
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Salt will attract moisture from the air. I had a box of rock salt in my garage that actually dripped enough water to make me look for a roof leak. Just something to consider. Don Young

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