The outdoor air isn't really relevant, the issue is human activity (breathing, showering, laundry, etc.) raising the indoor humidity when combined with low indoor temperatures causing surfaces to approach the dew point. Particularly external walls or windows that will be a lower temperature than the room as a whole.
At 70% RH and 15C air temperatures, the dew point is 10C - which could easily be achieved along the exterior walls of an older more poorly insulated house.
At 70% RH and 15C air temperatures, the dew point is 10C - which could easily be achieved along the exterior walls of an older more poorly insulated house.