While this is alarming in the context of ethnic cleansing in Western China, it's not really remarkable that foreign intelligence is operating in the US; what country on Earth does not have foreign intelligence agents operating in it?
Who else would bother?
Anyway, for a specific US example, Russia alerted the US about the Tsarnaev brothers. Anecdotally I heard from a South Korean about a visit in Japan from some official with photos warning him to stop associating with some North Koreans he'd met. Call me pessimistic but I think this is going on all the time everywhere.