A certain portion of the area below ceiling is enclosed by me with a wooden doors and lower part for material strorage as wall is available on 3 sides and ceiling above. The enclosed area painted by Asian Paints washable distemper using Asian paints stainer and is adjacent to the bathroom of the flat above. A few months back, I had observed oily stains in the portion all over the walls and a small portion of the ceiling in the enclosed space. I applied 3 coats of washable distemper to cover the stains. At that time I tried to diagnose the problem by making 3- 4 holes in the walls expecting some dampness / water seepage, etc. But no visible signs of the same. I checked the bathroom above and there too no visible defects. The plumber and mason were brought in and they said that this could be result of some discharge from the paint or sand used in plaster. Now the oily stains are again gradually showing up after 2 months.
Can you help me by telling what can these stains be? What are the cause and how are these to be treated?

Hi Fanfoskar,What country are you from> Sounds Like the UK, correct?
No water damage? No moisture? Just a oily stain? I agree with the plumber and mason. This doesn't sound like a plumbing leak but more likely a impurity that got mixed in with the paint or plaster. My bet's on then plaster. Why not remove the oily area and patch band over it.
Since this doesn't sound like a plumbing problem I'm moving 6this thread where =it will get the attention from other experts. Good luck, Tom

Well I too think the same. I hope you have seen the images too. ANd just for info, I am from Mumbai commercial capital of India

Please keep me informed on what you found. Have you remover the plaster and patched the spot yet? Cheers, Tom

I have not removed entire plaster yet. But the mason again stood by his statement that it is not water seepage. He too suggested removal of the paint, applying putty / POP and then repainting it. I will keep you informed Mr Tom

Please let us know what you're doing and if it works, Good luck, Tom