THE BELATED NEWS that a low-yield nuclear explosion, or some natural event uncannily resembling it, occurred last month somewhere around South Africa raises two unsettling specters. The first is that South Africa may be on its way to a nuclear bomb, though it must be said at once that there is no evidence except of a circumstantial sort. South Africa, which has never signed the nuclear non-proliferation treaty, is known to have the technological capacity and is believed to have the political motivation, and the explosion took place in its neighborhood.

The very thought that the ruling white minority in Pretoria would secretly equip itself with a nuclear bomb, especially after having promised two years ago that it would not, is chilling. It suggests a measure of defiance at odds with the conciliation that ought to mark South Africa's approach to its black subjects and neighbors and to the world beyond.

It is scarcely less painful to consider that the United States has spent five weeks intensely investigating this event and still cannot say just what it was, even whether it was a nuclear event, and who or what was responsible. There are explanations for the American government's bafflement and they are not insubstantial. Yet their collective implication is awesome: it may be possible for a determined and wily government actually to test a bomb and aovid detection.

With no clarification, there will be guesses that the tester was, if not South Africa, then Pakistan, or Israel, or some other nation that simply towed a barge into a blind place on the globe. But there may not be a positive identification. To know that a country has acquired a bomb is one thing; not to know for sure which country is another. The mind reels at possible crisis scenarios. The limitations on the capacity of intelligence thus demonstrated will have their own impact.

Should the administration have let the rest of us know about this event at once, if only to leave no doubt about its alarm over proliferation? The event took place on Sept. 22. On that day, you will recall, the administration was reaping a bitter political harvest from having released word of a Soviet "combat" presence in Cuba before it had all the facts in hand; it was being attacked precisely for loosing "premature" word. The diplomats were holding their silence on the grounds that South Africa could not be effectively challenged until the United States had nailed down its case. If there was cause for discretion then, however, there is now no escaping the need to express full concern and to keep the public informed about efforts to solve this deeply troubling nuclear whodunit.