We have rivulets that look like veins appearing around most of our windows. We assume that there is leaking somewhere. We have had a professional seal around the windows and treat them with Kills on the inside. But after a time the rivulets reappear.
We have parapet roof from 1926 and it has been sealed with silicone. Hard to imagine where water is getting in from the roof. I suppose that the windows were shoddily redone at some point and so the integrity of the casings is dubious.