The interesting fact is that there is very little speak of any other nation carrying out a nuclear test. It boiled down to Israel and South Africa, or a sensor glitch/weather fenomena. Kind of fun that the two nations vere the prime suspects although officially "nothing happened".
*Permanently uninhabited, but there's a research base there.
Prince Edward island is only 12 miles or so from Marion and visible from the base so it certainly did not happen in the waters between them.
Atomic tests have routinely been carried out with scientific observers nearby, I mean, it's sorta the point of doing one. And as for people noticing, I would imagine apartheid South Africa had their own equivalent of an Official Secrets Act.
Also, the fact that the overwintering teams are all listed as ending their time there in September strongly correlates to the date of the Vela Incident. So entirely plausible to send the winter team home a few days early and delay the the summer team.