Under the order, all those who live at Two Island Place are required to stay put and undergo coronavirus tests overnight.
The Centre for Health Protection said the patient in question, a 22-year-old man, had flown in from Serbia on September 12.
Officials said they suspect the man – who was vaccinated with Covishield in India in July after being infected with the virus there a few months earlier – was a “re-positive” overseas case.
He tested negative for the virus upon arrival here, as well during his mandatory hotel quarantine period.
He completed his hotel quarantine on Sunday and went for testing at a community centre the following day as required by self-monitoring rules.
That was when his sample came back Covid-positive.
Meanwhile, the centre said it was investigating two other preliminary positive imported cases, involving two women who had returned from Jordan and tested positive for the L452R mutant strain upon arrival.
Both were asymptomatic and had received two doses of the BioNTech vaccine.