Fingerprints, splashes of anything oily (they look like some type of splash mark, even sweat or water on a freshly sanded surface can alter how the satin is then absorbed).

The only way to repair it is usually to re-sand and stain again.
the spots do NOT look like typical grain blotching, but that some liquid splashed on the bare wood.

brickeyee ~ I agree, these are from something splashed on it. I appreciate your response, and was afraid that's what I'd hear, but I can do it

Sorry.

It is one of the hardest things about finishing wood.

Once the final sanding is done you need to get the surface stained as quickly as you can to protect it from contamination.

When I have had interior woodwork that gets even a few drops of rain or dew I immediately wipe down the whole surface with a damp sponge, then wait for it to dry to a uniform surface again (preferably inside).

Thank you again! I plan to work on this when the weather will allow for good ventilation, and I can clean and stain it right away.