I find this surprising considering that the ROC Army/Guo Min Jun/Taiwan Army should have more than enough personnel to man even such guard posts, since all local men who reach a certain age are required to serve a certain number of years in the military. This system of conscription has been in place since the Chinese Nationalists retreated there and the threat of mainland Chinese invasion to the island nation is as real today as it was in 1949.
I really doubt that the guard just left to "take a leak".