The trapped moisture cannot escape
ya i store it in a case, does that hurt it any?
I've seen this in the past with those not enlightened with the correct info.
What happens is, if you store your long guns in a soft case and leave them there, especially after the short bit of rain you mentioned, the moisture is now inside the case and on the rifle and has no where to go. Right against the blueing surface, there is no way the wetness can escape via air movement or evaporation. Just sits against the poor rifle/shotgun and corrodes it's surface. That is, if you do not take it out within a very short period of time to clean &/or oil it.
Soft rifle cases are meant for short term storage only. Preferably just back and forth from the range or hunting. Especially in your geographic location. Local rainfall is bound to have sea salt mixed with it. I can't think of a worse combination myself.
If you decide to use the rifle case for long term storage, this can negate rusting issues: Once home take the now empty rifle/shotgun out of it's case and place it on a dry, safe, flat surface so the gunmetal can adjust accordingly to room temperature. If it is actually wet right now, take the time to wipe all moisture & dirt from the surface with a soft dry rag. Take the empty case and keep it unzipped, now hang it higher up, like draped over the top of on an open door, while spread out like a gutted codfish after it's been salted. Ensure to leave the case up there to totally dry out, from just the house heating air. After a few hours should be fine in a normal house, but still check for surface condensation on the gunmetal, especially if the outside & inside temp's are at the opposite ends of a thermostat.
Once the rifle or shotgun has stabilized to household temperatures, you can now lightly oil the outer surfaces of it. Or disasemble it and then clean it,
then lightly oil it and reassemble.
If one rushes this procedure of basic care, you could have oil and small water droplets from condensation, on the gunmetal at the same time. This is because the gunmetal had not yet equalized to inside room temp's and it 'sweated out' moisture in the meanwhile.
Yes, you can ask those that know me, I am OCD about rust. I will tolerate a patina on antiques, & that's about it. Or corrossion pitting that has had the red sh@t removed professionally, such as seen on arsenal refinished Garands from WWII. Sometimes you just have to live with the minor pitting as sometimes seen inside the barrels of old blackpowder muzzleloading firearms. Or the slight surface corrossion caused by someone long long before you took it into your possession, via sweaty hands on the backstrap of an older blued S&W revolver from way back. Again, only on worthy & safe
antiques.