Air leaking INTO the house where that air is WARMER than the inside air MAY cause this problem. If the outside air is COLDER (hence dryer) than the inside air, you shouldn't experience this.
Now, you didn't mention anything about a fireplace or stove heater or space heater or anything like that. Again, these 'things' give off moisture.
Also, if the window in question is on the NORTH side of the house where there is no sun, the inside air needn't be that humid to cause this problem. Try circulating the air in this particular room or 'spot'. At the same time, if the window is on the SOUTH side of the house, then as it cools, moisture may form on it too. Again, circulate the air to see if it helps.
My home is very dry in Winter. And the front of our house faces South BUT we have a porch overhang. Fortunately, we don't have a moisture problem. You could buy a Hydrometer to see what your humidity level is. Check that w/the 'normal' levels for homes in your area.
My best to ya and hope this helps.
Jay J -Moderator
PS: God Bless America!