PVL-101 Kindral Kurvits in Kopli Bay Tallinn 24 May 2016.o Photo credit: Pjotr Mahhonin. At the time of the photo, this ship was part of Estonia’s  Police and Border Guard Board. Since then she and three other vessels were incorporated into the Estonian Navy. I don’t know if she still wears a stripe. She is the largest vessel in the Estonian fleet. 

Marine Link reports,

“The Estonian navy detained and boarded a Russia-bound oil tanker on an EU sanctions list on Friday, accusing it of sailing illegally without a valid country flag.

“The vessel, the Kiwala, has been under EU sanctions since February as part of Russia’s so-called “shadow fleet”, a term Western countries use for ships they say Moscow deploys to circumvent international sanctions.”

Estonian Navy was exercising a coast guard function, something the US Coast Guard might do.

The tanker was reportedly flying a Djibouti flag, but Djibouti said the ship was not registered with them, making the ship stateless. Which in turn means she is subject to the laws of any state that chooses to exercise the right.