Is the gutter secure or leaking in the outside ?
Any stained brickwork outside ?
Do you have an unvented tumble dryer in the house ?
Do you dry washing on an aired in the room ?
Is it close to your bathroom (with showers and baths in the evening) ?
Do you have trickle vents on your windows ?
It could be cold uninsulated walls.
It sounds like condensation to me, but difficult to say.
Hope this will help you