The block in Lido Garden was sealed off for overnight testing after one person living there was confirmed to have Covid-19.
“As there is a confirmed case recorded in Tower 2 of Lido Garden recently, as assessed, the risk of infection in the relevant area may be higher,” a government spokesman explained in a statement.
Authorities cordoned off the block at 8pm to test everyone living there.
They finished the operation at 7am, when the residents were allowed to leave for work.
The patient, a 55-year-old housewife, was one of three local cases reported by health authorities on Tuesday and the only one that could not be traced.
She started coughing on April 4 and had been to the Yeung Uk Road market in Tsuen Wan and gone grave-sweeping in Tsang Tsui in Tuen Mun during the incubation period.
The other two local coronavirus cases confirmed on Tuesday were her relatives.
There were also 10 imported cases, involving people arriving from India, Pakistan and Indonesia.
