I am not sure if you're obligated to pay US federal income tax or not (either non-US resident/citizen living in the USA or US's resident/citizen in expat).
If the IRS sends a formal letter to you, their intention is pretty clear that they found something wrong with your tax situation. I have had a couple of official letters from them for the last 4 - 5 years. Some of which I owed them money and I had to either pay (with penalty) or if allowed, file a amendment soon after. For others, they said I had errors in my filing and they gladly return a few dollars for me.
The first time I had to deal with them, I was surprised that the IRS is very professional, patient and courteous to me! I believe if they assume you're not trying to cheat taxes, they will help you to solve your case.
It make sense in cases where people get rich overnight (vested options, bitcoin gains or what nots) and they already spent a huge chunk of that income without setting aside for tax. If that is the case, they will soon realize they need a sizable portion of that overnight gain to pay tax soon.
The first time I had to deal with them, I was surprised that the IRS is very professional, patient and courteous to me! I believe if they assume you're not trying to cheat taxes, they will help you to solve your case.