US currency does travel overseas frequently, especially larger denominations as some countries require nothing less for exchange purposes. At least one of the most documented dollar bills on that website, A1473---0F, traveled from Illinois to Australia, then Philippines, then Korea, then back to the US, then New Zealand, then China, then Singapore, most of that between Oct 2003 and Oct 2005. Since the bill will pass through many hands that won't actually document its whereabouts, we have no idea of knowing if other bills that were undocumented had actually left the US, and then returned.