When I replaced my radiator, I also needed to top up the header tank after initial drives.
Let it cool down, see if it's still there and if so, get a sample out to have a better look.
If you don't have a syringe, then a cooking baster will do just as well....
Ideally, get as much of it out as possible so remaining coolant is clean, top it up, and check if more appears after your next drive.
If it disappeared when coolant got hot, then suggests some form of surfactant/detergent which as you've already speculated has a number of potential sources.
You can confirm this by heating up your sample BTW. If it doesn't dissolve, then could use a magnet to check if it's metallic?
Good luck!