Two things. Is the meter accurate? Im not saying yours isnt but when i do a reading i always like to double check. Because the meters Can go outta whack and need adjustment.

Now assuming the meter works in good fashion. You said water pipes are 20ft away that doesnt matter. If you have a leak water can travel. If you had a full blown leak youd know but condensation over time could cause the same issue.

Pet stains in theory could get your high reading if the pets urinated in the area fairly recently.

But you are left with two scenarios. One you get some fans out and let the floor dry out for a week and then retest. Or you rip up the floor in the area and inspect the subfloor.

Was your floor cupping? Why did you start sanding?

We found out it was the furnace output ducts in the basement. In the summer they were colder than the air around them and made moisture, in the summer they were hotter than the subfloor, and created moisture. The line of wet wood lines up exactly with the air duct and even turns a corner where the square duct branches off to go to another area. I am going to invest in some hvac wrap. Thanks.

Pee is hygroscopic; it pulls moisture out of the air because of the salt it contains.
Casey