Hi, I'm new here, and thanks for the opportunity to post a question. We had our white oak floors sanded and refinished 90 days ago with no stain and 3 coats of Bona Mega One water-based poly. In a few random spots, one inch in diameter ring shaped stains have appeared in the finish. At first, we thought it was from vinyl rubber furniture coasters, but later realized the stains don't match the footprint of the coasters, and some are in locations where nothing has been placed. The stain is definitely in, as opposed to on, the finish. Now I'm wondering if there a tool used in the refinishing process that could have contaminated the floor this way? What else may have caused this? Can it be spot repaired? Our contractor is, regrettably, giving us the runaround; promising to repair it, but not returning calls to actually schedule it.

Photo: https://photos.app.goo.gl/Mfc5wPefb5Y7WrUG7
https://photos.app.goo.gl/ztbXocHBuW1FSd1v8

These stains look like they came from the rubber casters of an air compressor.
It may have set in a solvent when stored and a minor amount of the component could have been deposited when the compressor was used for installing trim which is normally installed between coats of finish, usually before the final coat of finish.
If there are not 4 matching rings, the final screening may have abraded the other prints, or the other casters may have not been exposed to the same type of environment. The compressor may have been set down in almost the same place day after day during installation.
Does this seem plausible?

Yes, it seems quite plausible. They installed the shoe moulding the same day that the final coat was applied. There is one other place with a much lighter stain that is my fault. It is from a loop of clear vinyl tubing placed between a decorative ceramic pot and the floor to prevent scratches. This was 7 days after the final coat. Had no idea that could stain. It must take a very long time for Bona Mega One to cure.

Thanks to Pete's insight, I determined that the compressor, or whatever tool it was, has 3 feet (1 inch diameter) set in a triangle with 11-3/8 inches between legs. Does that ring a bell for any small hand carried compressor?