It's really hard to know what mode some of the new headlights are in as they approach you.
A quick flash is friendly, don't read too much into it. The wanna fight drivers are the one who flip on their brights for the last few seconds they are in front of you. Really a passive aggressive thing I guess.
I drive a F350, so my headlights are higher than some cars roofs. A Prius for example. I've passed many small cars, headed the same direction. Afterward, they pull behind me and turn on their brights. That's a mistake, I have exceptionally bright rear facing lamps, mounted high on a back rack, for illuminating camp sites, the truck bed, hunting camps.