Wasn't really much of an experience, but this seem like rain water were absorbed by the outer wall and the water flowed through the wall and appear at the ground floor ceiling.

check if any internal consealled water pipes, check if the wall near toilet, it could be toilet fr upper floor, check upper floor what they put up there, building a mini fish pond or what. you never know.

i have seen ppl use thier balcony and use it as a fish pond! The pressure of water forced the water to the floor concrete.

what is below of the leaking area? is that Kitchen zink? any tap?
Because Kitchen area normally should have piping need the wall area and link to other part of the wall.
You could imaging yourself as your unit design are similar to the unit above yours and what the possiable cause...

But this is the unit above you given problem right?

Mostly cause by piping leakage inside the wall. Cure it fast before spreading throughout the whole wall.

Poor thing,

It could be you pipe but wonder why would anyone connect pipe from top into the unit?

To check if the leak if from main pipe is easy. Turn off all tap and make sure no one flush the toilet. Go to the meter see if the meter run. Watch over 5 minutes or so. Take a puff if you smoke.

There is a possiblity it comes from bathroom. In the bathroom you cannot see water mark on the bathroom wall because it is tiled but you can see on the other side of the wall and if the other side is outside then sooner or later fungus will grow. It could also be the bathroom floor tiled not properly waterproof and is very common but should show water mark on your ceiling toillet if not the nearby room ceiling, not ceiling far away from the bathroom. This is because the bathroom floor are waterproof and the water cannot come down directly to your ceiling toilet and flow to the nearby room ceiling that is not waterproof.

Aiya, get a good plumber la, contractor cannot pakai one! only know how to make money out of people misery. Plumber is the one who do the job, rectify problem, contractor is the one who talk other do job and another pay and he counts money. A good plumber with experience can tell as they see these things so often and this is their rice bowl.