As Sharpshooter so stated... any thing you fire, is technically corrosive. If you fire off caps, and then don't clean the area.. you probably will over time find the start of corrosive forces working. The rust is normally not too wide spread from caps.

Leaving a rifle loaded for extended periods of time. All depending on the environment you hunt and live, whether it will effect the accuracy and over all function of the load itself. In Northern Wisconsin where I live its cold, wet and humid. Not a good combination for rifles, powder, etc. When I hunt in snow, I always unload at the end of the day, clean the rifle, and start new the next day. But I am sure there are others that do not. Its all a matter of preference.

I have had misfires from leaving the rifle loaded, and even hang fires or half fires. So again, do what you think is best. IMO and only my opinion, 3 weeks is too long to leave a rifle loaded and unfired. Were it me, I would answer a lot of questions for myself, next time in the woods. I'd pick out a spot on a stump, get a good rest and let fly. I'd see if it hits where it is aimed, and then swab the rifle clean. After that I would check it for all the rust and fouling possibilities that you currently question. Knowing what I can expect is a great satisfaction.