The leak appeared in the kitchen. I had cut a hole in the ceiling because the water tank sits above this so there is no access from above! The water appears to be trickling out from the joint indicated in red. It then flows down the light blue stained part of the pipe. Its a very slow leak, about 1 drop every 4 to 5 hours, it seems.

RED – The joint that appears to be leaking.

BLUE – The connected pipe section.

Below is a short video showing the area in the ceiling.