The situation discussed in the article was with regard to Stingray-type devices placed at foreign embassies which are considered foreign soil. The FCC doesn't regulate embassies any more than it regulates Beijing or Moscow.
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.
Embassies are not "foreign soil". The Russian Embassy in London, for example, is not a Russian place, it's a British place that just happens to have a Russian Embassy building on it. Russians there are not magically exempt from British law.
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.
> 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.
Because not every place is a shining example of liberal democracy like the United States.
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.
To track who (or at least what devices) are in the immediate vicinity of the embassy and when. Patterns in that could easily be useful for catching physical surveillance at the least, as well as catching placed/planted devices that check in that way.
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.
The loud music would draw a political rebuke / protest response, and then after if it didn't cease, it would plausibly draw some kind of tit for tat response in the other nation. The US can expel diplomats and isolate an embassy (eg cut power, water, etc), essentially making it non-maintainable (inhospitable) as a position. It could also surround it literally, effectively sealing it off to access, preventing the ability to leave (with predictable consequences).
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.
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.