US citizens and resident aliens must report their worldwide income, meaning income from all sources within or outside of the United States. The IRS receives tax filings and payments all year round and from all over the world. If you derive income from sources in either Frankfurt, London, Paris, or Beijing and happen to be working there too, you have the fortune (or misfortune if you have an unpaid tax debt) to have an international IRS office nearby.
I would normally not recommend dropping in at a local IRS office. You can usually get what you need more quickly, and with less hassle, by using the IRS’ website or by calling and speaking with a customer service representative if you don’t have a tax attorney. However, if I worked in one of these four cities, I might give the international office a try. They are located inside the United States embassies (or consulate in Frankfurt) and I would bet they do not get a lot of foot traffic. Be sure to check the hours of operation here because they vary office to office, or visit the individual offices for more information: