At roof level, how do the houses join together - eg is there a 'valley' where the properties meet at roof level? If so this can be blocked with leaves and stuff, causing rainwater to flow out of the gutters and into the wall cavity.
Are there any cracks in the brickwork or the render, that water could penetrate along? (look both sides of the property line)
Do you know what the adjoining room is ie could it have a bathroom with a leak?
If it is not near a window then can the problem originate with the join between chimney stack and roof tiles?