Those conditions are ripe for condensation buildup on a daily basis whether it runs or not.
35% humidity during the high temp of the day, can become much higher during the night. When the temperature falls to a point where the moisture can't be held gaseous, then fog or condensation occurs.
If you are getting dew or frost on surfaces outside overnight, you will get condensation buildup inside your engine, unless you have a heated garage. Once water gets in there, you need 212 degrees to boil it off as steam. Or, you need to expose the water to a low humidity atmosphere for it to evaporate into it and then be expelled into the atmosphere.
Honda used the air filter box vacuum to draw the vapors from the crankcase. The partial vacuum also helps speed the vaporization of the water and sends it through the engine combustion cycle.
A tube vented directly to atmosphere can suck in a fresh humidity charge every day when the air expands and contracts with temperature changes. Anytime the heat of the engine inside is greater than that outside, condensation can occur inside the engine. In your case, you have no positive crankcase ventillation (PCV).
I seem to recall older car engines before PCV and recirculation into the induction system. They had at least two outside vents. one near the bottom and one near the top of the engine. This would allow for some kind of flow through the engine which would aid in vaporization. The CB550 only has the one vent, I believe.
In the winter here in CA, my Cb550s have a hard time getting the water and vapor out of the engine. About once a month during rainy season, I'll notice what looks like brown mayonaise on the dipstick body during and oil level check. I learned that this usually means the breather element in the filter box is soaked with condensation, plugging of the breather. A day or two after clearing, drying, the foam and a good 25-30 minute stint on the freeway, and the mayonaise goes away.
If you desire to keep the open breather tube, you might consider plugging it during times of non-operation. Or, perhaps connect a canister of dessicant to the tube, during storage, to keep the air contents dry going into the engine.
I also speculate that your oil cooler is exacerbating your problem. It is likely keeping your oil at a good operating temp. But, for the temperature conditions in which the engine is operating, the added mass is over cooling the motor as a whole, and not allowing the engine to achieve temperatures high enough to effectively vaporize the water vapor. You could also try covering your oil cooler and blocking the airflow through it. I can't believe you need an oil cooler unless in very hot weather and running the engine very hard. 50 degrees F ambient temp is hardly a taxing environment for that motor.
Cheers,
