During the Discovery Expedition (1901-1904) to Antarctica, a pre-arranged message point was set up at Cape Crozier so that relief ships would be able to locate the expedition. What exactly was that message point? A flag semaphore point? A hut where written messages were left by the expedition team to the relief ships?
The usual method was to build a cairn of large stones, rather than a hut, and put written messages in watertight containers into it. This doesn't require transporting materials to build a hut to Antarctica, and is much less subject to being blown away by storms. Source: practices of the Franklin expedition to the Canadian Arctic.
A flag semaphore point would have to be manned, and that would have added considerably to the manpower and supplies required for the expedition.