The Stingrays found on K Street (far from Embassy Row) and some bridges were more likely US government operations.
So the question comes back to are there non-embassy, non-US government Stingrays deployed and how to find them.
If an embassy started blaring very loud music (or a siren), can the US do anything about it?
Likewise, if they started emitting strong microwaves at people, can the US do anything about it?
It seems like there must be some limitation to what you can do from the embassy to people outside of it.
The strong microwaves would be treated as an act of war: an attack on Americans, on American soil, by a foreign power. If it didn't immediately stop, the US would invade that foreign soil and do whatever it decided was required.
But that's a drastic action that won't happen before many other options have been tried.
Edit: To expand on that, some examples:
If a new device shows up and is always present, particularly if it always has about the same signal strength or doesn't appear to move, that indicates a connected IoT device of some sort, and if you're concerned about espionage you may want to take steps to identify it.
If a particular device shows up for 8-12 hour shifts at varying times, but there are no businesses, etc. that would have that kind of attendance pattern, who's carrying that device? An investigator on-site who's also brought a personal device along?
Heck, if you're in an OnStar-equipped vehicle even if you don't have service, your vehicle may show up as always on, or at least may ping regularly.
I'm sure appropriate data mining techniques could pull a surprising amount of information out of the kind of info gathered from these devices.
However, in practice diplomacy is impossible without affording Ambassadors, their staff, the places where they live and work and so on, broad immunity to normal civil law enforcement. Eventually this was formalised as the Vienna Convention, and the current iteration of that convention is the state of the art as far as relations between most countries are concerned.
As a result Convention signatories do NOT on the whole search embassies of other signatories in their country. But it's not because the embassy in any sense isn't in their country.
For example the US government absolutely could tell the Russian mission to all shove off back home, they would be entitled to a "reasonable" amount of time to leave, and then the Ambassador (if he has foolishly remained) is just a Russian citizen in the US without immigration papers, the same for all staff and families. The embassy, the homes, and other facilities are all just ordinary buildings able to be searched by police, parcels sent to the embassy become just ordinary parcels which may be opened, examined, redirected or destroyed as appropriate by the USPS. The Americans would never choose to do this, because diplomatic contact with Russia remains essential, in anything short of total war, but legally they absolutely could.
Why is that?
Diplomacy requires representing your country, which sometimes requires advocating against the preferred policies of your negotiating partners. In some places, if you were subject to their laws, that would get you killed.
There has been zero evidence presented so far that it is in fact foreign governments doing any of this. More likely it's the vast cabal of Federal agencies in the greater DC area all disregarding the law because nobody in the US Congress has the will to rein them in. And of course for all we know, the espionage may be entirely legal courtesy of some clandestine national security FISA sign-off.
The US Government system is made up of dozens of extremely powerful agencies, and they are at war with each other at all times for power, information and budget. Sometimes that war is cold, sometimes it's warm.
Simply put, they're all spying on each other and all aspects of the system that is meant to control them (while they attempt to control the system instead).
If there was a single city in the US that foreign agencies would target, it would undoubtedly be DC. The abundance of embassies in the city make it a very easy target.