I agree with hamibabe, my money is on the roof terrace tiles or grout lines, also just because the damp is next to the vent it may be leaking 6 ft away and if the ceiling has a slight slope the vent area is the lowest point of the ceiling that is where the water will be lying.
regrouting and sealing the outer edge of the roof terrace will prob cure it
try ML but good luck with that!!!