Why the delay
Water entering through a terrace does not fall straight onto the ceiling below. It spreads through the mortar bed, travels wherever it meets least resistance, and surfaces at the point where the slab lets it out.
That journey can take days or weeks, and the mark often becomes visible when the good weather returns and the water starts evaporating from within. Hence the impression that the leak has nothing to do with the rain.
The mark is not below the hole
It is the commonest starting error: looking for the entry directly above the mark. On a terrace the entry point is usually several metres away, almost always around the perimeter.
The three usual suspects are the floor-to-wall junction, the terrace skirting joint and a poorly sealed drain outlet. In properties with tiles bedded on mortar, a single open joint is enough.
What gets checked, in order
First the drain outlet: lift the grille and see whether the seal around it is still flexible or has cracked. Then the perimeter, looking for fine fissures where the floor meets the wall.
Then the joints between tiles, which on a sun-exposed terrace have spent twenty years expanding. And finally the downpipe, which can be leaking inside the wall itself without leaving any trace on the terrace.
When it is the neighbour's terrace
In a block, the terrace causing the mark on your ceiling is often the one above, and the repair is no longer yours to make.
A written diagnosis with the entry point identified is what lets you talk to the neighbour or the community without the conversation turning into an exchange of impressions.
Why November brings it all out
The first serious autumn rain arrives after five months of sun that have opened the joints. The water gets in, takes its time, and shows up right about now.
Marks appearing in November are almost always the result of October. Repairing before the next spell of rain is what stops December's coming in through the same place.
