Had the same trouble with an entry level Stoeger O/U shotgun. Had it out early in the morning and it got a coat of dew on it. Somehow it ended back in it's case without getting much more than a rub down. Took it out and it was speckled with rust.
Took a few minutes with some 0000 steel wool and rem oil, and it's like brand new again.
Took a few minutes with some 0000 steel wool and rem oil, and it's like brand new again.