My only take here (besides the certainty that the shadowy hand of the FISA FISC court is not behind all this) is that it doesn't look like another ICE raid.
I don't think anyone, least of all OP, is suggesting this is what happened.
Everything is rank speculation at this point, but if I were going to speculate I'd speculate that Wang suddenly left the country of his own volition and the employer began investigating his disappearance and stumbled on things that led to them calling in the FBI.
That's still purely speculation, but it would account for the sequence of events and the manner of investigation way better than any ideas about him being "vanished" by the US government.
You don't know if its detained, you don't know if is in the territory of the USA, you don't know anything....If it's a US Citizen would be espionage because?