water stains on kitchen ceiling along side chminey brickwork

Hope someone can give me some pointers as to the most likely cause. We have a post and beam house. The kitchen has a high ceiling but no attic space that can be accessed. The brick work of the back of a central fireplace is in the kitchen and was pained white by the previous owner. Where the brickwork meets the ceiling there is about a qurarter inch gap which you can shine a torch up to the brickwork slightly past the kitchen ceiling. We are noticing brown stains at the upper portion of the brickwork in a random areas with not apparent lines and very small spots of dried water on the top of a pantry cupboard that abutts to the wall. I have looked at the roof and the flashing around the chimney for any obviouse signs of leakage, but looks ok and was done redone about two years ago. Some of the mortar in the chimney is loose and could do to be repointed.
My question is. Is it more likely that the brown stains on the brickwork and apparent brown water dropplet stains ( 5 o 6) are more likely due to the warm air moving up from the kitchen to the space between the ceiling and roof, condensing and then dropping back down.or accumulating on the brickwork as it rises, as oppossed to leaks from the roof or possible the loose morter in the chimney?
Can water damage come from the morter at the exposed part of the chimney or would this just go down the chimney?
Intend to address the repointing needed and also add some more seal to the flashings just in case but would appreciate any feedback.
Thanks Chris
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