I have been told both my Freyja and Finn are sabino. I don't know a lot about how the gene manifests, but Freyja has stray whites like that throughout her coat in the black areas. She also has the similar "blotch" on her blaze - and her lower lip/chin is white as well. Finn at the moment has very few stray white hairs - a total of like 20 over his entire body, and then he has the small white belly blotch, but I have been told his tail is a giveaway in the sabino genetics. It's about a million different colors. That and his white socks come to a "point", along with the belly splotch, are also indicators, so I am to expect his "stray whites" to spread over the years to eventually be everywhere like Freyja's are.
Now, this is all just conjecture from horse color gurus I have spoken to. Neither Freyja or Finn have ever been actually TESTED for the sabino - just so you know.
I don't have a good pictures of the whites spread throughout Freyja's coat, I'll run outside and try to take one, but it's similar to the picture you showed around the base of Zierra's tail - only it's everywhere (and spreading more every spring) on Freyja.
I should also specify that Finn and Freyja are both part Shire - a breed known to carry the sabino gene. I personally don't honestly know how to tell the difference between sabino vs rabicano, to me the look very similar.
Finns jagged socks and belly spot:
Freyja's blaze splotch: