Stainless Steel comes in many forms. 300 series Stainless is truly stainless. 400 series Stainless Steel, which most guns are made of, can show some rust. Most guns are 410 Stainless. They are "stain resistant", but not rust proof. Use some fine steel wool (000) and a light oil ( I use Kroil) to polish the spots off. You can use any light oil, like 3 in 1 light household oil. I am sure there are other methods. A polishing compound would probably work, but you may end up with spots that are shiner than the rest of the finish. Do not use sand paper, unless it is very fine. I use the steel wool it seems to work well. I keep an oil soaked one in a zip lock bag and use it to polish up any gun (even blued) that may have any rust on them. After you get it "rust" free, give it a coat of a good rust preventative, do not forget to coat the receiver too. Also - do not forget to look under the fore end for rust. I like Break Free , but Rem Oil is good and some even like WD-40. I think the first two I mentioned works a little better, but WD-40 is better than nothing, even better than some so called rust preventatives. Do not forget to keep the latch/lug free of oil. If you get any on there, wipe it off before your next shooting session or hunt. Since you say you clean often, just wipe it down when you clean your gun with your favorite rust preventative and you will not have any issues with rust. The receiver you have is nickle plated, not Stainless Steel. If you get any rust spots on the receiver, you can use a chrome polish on it. I have not tried the steel wool on chrome, but it would probably work on that too. The only SS receivers have come with either a 45-70 or 444 Marlin barrels as Ultras within the last year. Good Shooting and Good Luck.
