With 40% humidity, I doubt you'd ever see rust form on guns that are wiped with a oily rag every few months. In my house, the humidity ranges from 50-60% in the summer to 40-45% in the winter. I always wipe a gun off after I touch it. Some guns don't get wiped off for a year or so if I don't handle them. I have never had rust form on any firearm.
dew is your biggest enemy. Humidity is relative to temperature. Temp goes down, relative humidity goes up. If air circulation is poor, some thing will change temperature quicker - it will get wet first.
I have heard of rain inside of our shooting trailer - Warm/humid air went in during the day, but the walls/roof hit dew point overnight.