What you see from below
A stain on the ceiling directly under the bathroom above, usually near the shower area or floor trap. It gets darker an hour or two after the neighbour uses the bathroom. White crystalline deposits (efflorescence) form at the edges. Paint bubbles and drops off. In advanced cases there is a steady drip.
What it is not
If the ceiling is wet at all hours regardless of use, suspect a supply pipe. If it only leaks when it rains, look at the balcony, the external wall or a window rather than the bathroom. The test confirms it; the pattern points to where to test first.
How it is repaired
Properly, the tiles and screed in the wet area come up, the slab is cleaned, a new membrane is applied and tested by ponding before the floor is reinstated. Coating over the tiles or sealing the grout is a temporary patch that usually fails within months. Once the source is fixed and a monitoring period passes dry, the ceiling below can be reinstated and repainted.
Report it with photos and you get a case number immediately. We inspect both units and issue the written report your management needs.