Particles showing through the polyurethane over my newly-sanded hardwood floors
 

Posting this for someone who said "we lightly sanded and wiped down with mineral spirits in between coats of polyurethane, but since we didn't use the tack cloth (which I just now told her about) before we polyurethaned or between coats of polyurethane, I doubt that we were able to remove all the dust." Now there's tiny spots showing throughout the polyurethane that's probably the dust that didn't come up.

Is there anything they can do to that coat(s) of polyurethane to remove those obvious areas of small dirt particles within? Lightly sand (maybe with the finest-of-grit sandpaper?) and then polyurethane over that once the dust particles are gone? Any other ideas?

That would be my suggestion.

You could use a very fine sandpaper and then use a polish and skip another coat of urethane.

Are you sure they are not air bubbles?

They checked last night and it's definitely not air bubbles. It looks like grit.

Someone mentioned to them that they might be able to just buff it out. They'll give that a try.