Casing a shotgun over long periods of time in not recommended. Besides having the lube drawn off where the lining contacts the weapon, some cases act like sponges, and ambient moisture wicks onto the metal.
If you've rust showing on the outside, chances are there's more hidden damage.
Places oft ignored, regretfully, on 870s are inside the mag tube, inside the receiver, and in the forend assembly between the wood and metal. The vents under the rib are also prone to corrosion.
Complete disassembly and deep cleaning is a good idea.
Wiping on a heavy coat of oil, CLP or SLIP, letting it sit for a few minutes and wiping off the excess is a good way to protect the metal.