Looking at the image, it appears the wallboard on the inside was wet all along the vertical edge. It could be a roof leak dripped water on the safe where it ran down into the door. It could be someone at the factory left the board lying on its side in a wet place or left a pallet of it out in the rain, wetting the edges of the boards in the middle of the stack before they were assembled into a safe.
The arrow pointing to contact rust in the photo is where there clearly was contact against something when water was present, causing the sharp-edged shape. Wet wallboard would do it. Recessed behind it is an area with a less sharp edge to the rust, suggesting the air was highly humid in that area, but the humidity dropped as you got further toward the center, due to mixing with drier air, so the rusting stopped. If acid in the wallboard is responsible, then I would expect the rust to be much more uniformly distributed all over the steel.
The GoldenRod or a light bulb may still solve the issue if water is involved. How they work is commonly misunderstood. Long ago (see Hatcher's Notebook from about pages 345 through 350) it was worked out that relative humidity (RH) holds the key to rusting. Even a bore that had corrosive primer residue in it was found not to rust aggressively until RH became about 68% or a bit higher. It doesn’t matter if it is 68% RH at 50°F or 68% RH at 100°F, that’s the threshold. Temperature difference alone doesn’t make the difference.
Relative humidity is just the amount of water dissolved in air as a percent of the maximum amount of water the air can dissolve. What makes it "relative" instead of just some absolute number of grams of water per cubic meter of air or per kilogram of air is that the maximum amount of water air can dissolve changes with temperature. So, the percentage is relative to how much water air can hold at a particular air temperature. If you can raise the temperature without letting more water into the air, the RH goes down because warm air can dissolve and hold more water than cooler air can. If the air inside and outside the safe is 100ۜ°F and 80% RH and a 25W heater can raise the temperature inside to 110°F, without added water, the RH drops to about 60% and rusting becomes much less aggressive, even in the presence of salt.
That’s why the GoldenRod is advertised to reduce humidity even though it removes no water. All you need to prevent rust is for the air to try to hold onto the water harder than the steel or salt tries to get hold of it. There isn’t actually exact RH at which the salt rust simply ceases because the whole thing is a giant equilibrium condition shift of rates at which water molecules attach to steel and get dried by off by air wanting it more. Instead, ability to rust fades as humidity goes down, first rather sharply (at a different RH for each type of salt) and then more gradually until you get to about 30% RH, below which air is so aggressive at gathering water that for all practical purposes steel can’t rust at all. However, 30% would dry out and crack wood stocks as well, so there really is such a thing as too much of a good thing where lowering RH is concerned.