did not reveal its sources and methods, at least in the BBC article, used to detect the "rock radio".

Since the transmission signals may have been very short range (don't know that, but that is an inference), signal intercepts may not have been the key. If the FSB surveils adversary diplomats as well as the KGB did, investigation and surveillence may, indeed, have been the key - too many UK diplos hanging around a location punching their hand-helds.

In any event, the 2006 incident seems closer to what m.c is looking for - simply updated in technology.