Sputnik just offered official "explanation" for the helicopter flyby above Russian embassy several days ago. Imo, someone was simply harassing them.
https://sputniknews.com/us/201803171062625672-us-helicopter-russian-embassy/
Here's Russian embassy tweet dated 13th of March:
https://sputniknews.com/us/201803171062625672-us-helicopter-russian-embassy/
The mysterious chopper that flew over the Russian Embassy in Washington with its searchlight on last week appeared to be a law enforcement helicopter on a mission, a source from the Federal Office of Civil Aviation told Sputnik.
“It was a law enforcement helicopter, operating with the knowledge of the Air traffic control services. It was flying over that area by order: they were either looking for someone or responded to a call,” the source said.
Last week, the Washington police offered a potential explanation, saying that one of its helicopters was dispatched to an area north of the Russian Embassy in response to a possible gun robbery, The Washington Post reported.
Here's Russian embassy tweet dated 13th of March: