The estate in question is block 8 of Nan Fung Sun chuen.
Former Eastern district councilor Derek Ngai Chi-ho said the signs appeared near the doors of residents. There were ticks near the door bell buttons, residents found.
They were worried that they were signs left by burglars during their preparation.
Ngai said he was not sure if the ticks were a joke or really left by burglars, but reminded residents in the neighborhood to pay extra attention to strangers.
The estate's management company put up a notice, saying security and patrols have been stepped up. It has also reported to Chai Wan police and crime prevention unit in the Eastern district. Police sent officers to the scene for investigation.
Countries such as Singapore and Israel have already started to administer a third jab to some citizens but Hong Kong is still conducting studies on such a move.