The smell in your house is sewer gas. Homes in North America by municipal building code requires vent pipes to be installed in all buildings with sewage systems. In HK as in the mainland, there is no requirement that the vent pipes be piped outdoors, to save money in lieu of running pipes 100 floors up, builders simply vent the sewage gas back into the occupied space.
Those little drain plates in the floors are vents, what comes out of them are the stench from the rotting sewage gas and whatever other chemicals are in the pipes.
The best thing to prevent the gas from backing up into your apartment is to either tape off the hole or get a piece of rubber about the same size as the drain plate, apply some silicone sealant or gasket material, place the rubber piece underneath the drain plate and reinstall the screws.
The smell will no longer backup into your unit, besides smelling bad, sewage gas is bad for your health...
Drain-waste-vent system - Wikipedia, the free encyclopedia
http://www.askthebuilder.com/Vent_Pipe_Video.shtml