Well, no one expects to be questioned at the US border when flying to the US for a simcha or visit...by the IRS, about back taxes.
This month's, "Journal of Accountancy" has a rather scary article about this very issue. First and last paragraphs are the key:
Taxpayers traveling to the United States with unpaid U.S. tax assessments can be detained at the border, questioned, and flagged for follow-up enforcement. If a taxpayer has an unpaid tax liability and is subject to a resulting Notice of Federal Tax Lien, the IRS may submit identifying taxpayer information to the Treasury Enforcement Communications System (TECS), a database maintained by the Department of Homeland Security (DHS).
The database allows the DHS to identify taxpayers with unpaid tax assessments who are traveling to the United States (Internal Revenue Manual (IRM), §184.108.40.206).
A taxpayer who resides outside the United States may not be aware of outstanding federal tax liabilities if the address on record for the taxpayer is outdated or otherwise incorrect. Consequently, tax advisers with clients who reside outside the United States should ensure that the correct address for the taxpayer is used on the client’s returns and, if the client no longer is required to file U.S. returns, that the IRS still is able to contact the taxpayer about previously filed returns. Taxpayers should be advised that a failure to keep the IRS apprised of a change in mailing address may result in an unwelcome—and potentially embarrassing—surprise when the taxpayer seeks to enter the United States.This could apply even if you DON'T owe money, but the IRS thinks you owe money, audited you (without your knowledge if they couldn't contact you), and then...while trying to visit...
So, make sure you file regularly, and make sure they have an address listed for you, which will actually reach you.
Remember the IRS motto: "We're not happy, till you're not happy..."
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