In Hong Kong, US aircraft carriers are like migratory birds - they come and go each winter.
This year, however, the giant flat-topped ships steaming through the mists of the East Lamma channel have been conspicuous by their absence.
Unusually, the United States did not seek Beijing's permission for even a single visit, even though two carrier strike groups have been operating in the western Pacific in recent months.
As reported in the South China Morning Post, a scheduled visit last October was scrapped and no request submitted. Officially, the US Navy said the reason was 'operational' rather than anything else - the same reason offered for their absence in more recent months.
Yet diplomatic observers and military analysts across the region wonder whether the reason has had a political element, too - some note that it was hard to believe Beijing would approve a visit, given the just-thawed frost that had fallen on the Sino-US military relationship in the past year, even if visits of lesser ships were permitted.
For decades carrier strike groups have been the ultimate symbol of US military primacy in East Asia - and regular visitors to Hong Kong since the 1960s. But now they also represent a strategic presence that must operate in an ever-more complex environment presented by China's own military rise.
Beijing, after all, has repeatedly expressed concern about the continued presence of US aircraft carriers off its coasts over the past year as Washington prepared to send the USS George Washington into the Yellow Sea following North Korean attacks on South Korea.