It makes sense to believe the USDoD has the entirety of E and SE Asia area under constant radar, SIGINT and satellite observation for military and security reasons related to Russia, China and N. Korea. In fact, I can pretty much tell you where the observation and listening posts would be, within 100-or-so mile radii. (Edit: What proof do I have of any of this? None. Just educated suppositions. It's rational, more than a lot of other speculations lately. It makes sense to think the US knows more than they let on...I'd wager China does as well.)
However, such disclosures would violate security protocols and reveal the capabilities of US intelligence and surveillance, to say nothing of revealing what has to be the worst-kept secret in the world...we spy on nations we do not trust, even as we affirm that we are on friendly terms. They know it, we know it and everybody knows everybody knows it. They do it too...spy on us. It's good foreign and intelligence policy.
So, when the plane shows up...the United States government will play coy. "So, that's where it disappeared to!" as if they didn't know.
I wouldn't be surprised if we were actually preparing for an assault action to recover the plane, hostages or bodies; probably not alone though but as part of a joint action based on limited disclosures.