According to this Russian vexillology website [1] this flag was used by the Russians of Latvia in political demonstrations.
While the shape of the flag reflects the affinity to Latvia, the color corresponds to the purple of the "non-citizen" passports issued by the Latvian government to "non-citizens" (see en:Non-citizens_(Latvia)). Use as a common symbol of the Russian community has been rare.
Own work, based on File:Flag of Latvia.svg, which is PD.
Hierakares