This was new to me so I ran it by our R&D team. A couple possibilities they came up with:
1) It could be surfactant/dispersant leaching. (Both are materials found in stain.) Could have happened if it was applied in a humid environment or the humidity had leached it to the surface. Some mildewcides do the same thing. Generally only happens when things are applied in a humid environment, then things heated up to cause that osmotic pressure, bringing things to the surface. While necessary for stability in the can, many surfactants and other soluble chemicals leach out of coatings - or wood - when it is subjected to condensing water vapor or dew. It is usually EASY to wipe these away and be done with the problem. If they are stained over, they could look similar to what is observed in the picture.
2) One more possibility... zooming in on the check/crack on the right side, note that the collected stain is wrinkled. Note that the spots are also wrinkled. This looks like an oil based stain that was sprayed on a ceiling and dripped but was not back-brushed or absorbed into the wood.
To tell the difference between the two, a simple scratch with a fingernail should tell if it is stain (will be hard) or surfactant leaching(will be soft/waxy and fairly easy to damage the spot).
So, try that test and see what happens. Once you've done that, feel free to contact me directly and we can walk you through some fixes.
Hope that helps for now. Have a great weekend!
--- Charis w/ Sashco - www.sashco.com/log-home - firstname.lastname@example.org
Yay for no growths! You can likely remove those spots with a little bit of mineral spirits applied right there. It should dissolve the surfactant. That said, it could leave spots if the underlying wood had yellowed over time. Do a quick test to see what will happen. It may require that you remove those spots, do some spot sanding, and re-stain / clear coat to get a match with surrounding wood.
Hope that helps some. Feel free to contact me directly or continue the conversation here if you have more questions.