Jeff,
I am a home inspector and I ran into this exact situation a few days
ago. Your problem sounds like a moisture problem in the attic. Yes,
the moisture from your bathroom is finding its way into the attic even
though you don't have an exhaust fan; the warm, moist air will find
its way into the attic around light fixtures, cracks, etc. When the
moist air comes into contact with the cold roof decking the moisture
condenses and leaves the decking wet (just what mold needs to grow).
The decking on the north side will naturally be colder than on the
south side so you will find more mold there.
Roof vents and gable vents are not the best combination because they
will kind of short circuit the ventilation of the attic. As warm air
exhausts through the roof vents cool air will be drawn in through the
gable vents causing the air in the attic to not be ventilated
properly. (Air should be exhausting through the gable vents, not be
drawn into the attic through them.) Your best bet may be to block off
the gable vents (screw a piece of plywood over the inside of the
vents) and install soffit vents; then you will get proper flow of air
through the attic from bottom to top and the air in the attic will be
vented much better.
You should consider adding an exhaust fan in your bathroom and vent it
properly. You can vent it directly to the exterior or you can pipe it
to a point close to one of your roof vents (don't place it so it
blocks off free flow of attic air through the vent). You may also want
to do the same in the kitchen. Also, make sure your clothes dryer
vents to the exterior for obvious reasons. Lastly, if you are using a
humidifier either stop using it or make sure it is set no higher than
35% or 40% RH (relative humidity); you might even want to consider
using a dehumidifier.
You may want to hire a home inspector to look things over but I
suspect he will tell you the same thing I have told you for free.

Bruce