At the airport, two stations — one for bracelet attachment, the other to sign a quarantine contract — reinforced what she already knew. In recent weeks, there had been a spike in coronavirus cases in Hong Kong, primarily involving people returning from abroad.

Those under quarantine could serve out their two weeks anywhere in the city, so long as they wore their wristband and stayed within the perimeter they registered on an accompanying app.

“I don’t mind,” Ms. Lalwani, 19, said over WhatsApp as she settled into her hotel, “if it means that it will keep both myself and everyone else that much safer.”

If she violated the terms, she could be shipped off to a government quarantine facility­. She would also risk a fine of around $645, six months in jail and social media retribution. It was the online mob that worried her most. She was familiar with its loose relationship with facts.

The coronavirus villain in the Ikea bed who had “cut off” her wristband, according to posts on social media and several news outlets, was a friend. The young woman, a fellow Hong Konger studying abroad, had actually never been issued a wristband, Ms. Lalwani said. An interview with the 19-year-old design student confirmed this; she said she had flown in from New York before the wristband program applied to arrivals from the United States.