I recently learned some lessons about gun safes that I should have already known. My safe was in the garage attached to my house for several years. This garage is semi-air-conditioned. I had no dehumidifier in the safe and never experienced any problems with moisture. Last summer I built a detached garage and before it was insulated or air conditioned I moved the safe there. A few months later one fall afternoon I opened the safe and found all my guns were covered with condensation. Water was literally dripping off of them. Naturally I panicked and installed not only an electric heating element and a rechargeable moisture absorber, I also upped the priority of getting the garage weatherized and air conditioned. What was happening is the same thing that rusts out fuel tanks that are stored with air space above the fuel.
In an unconditioned space, temperature cycles up and down each day, and relative humidity along with it. Air can hold a lot of moisture when warm, and that moisture condenses out when the air is cooled enough. At the same time, objects expand and contract. Objects that are hollow and unsealed breathe air in and out as they expand and contract. So here is your gun safe sitting in an unconditioned space in a humid climate: At night when the air is cool, the volume of the safe is smaller, and the guns are cooler. The next day, as the temperature increases, outside air picks up more moisture and as the safe expands it sucks that moisture laden air inside. Immediately on contact with the still cooler guns some of the moisture condenses on the guns as liquid water. Later in the evening as the temperature again falls, the safe contracts and some of the air is expelled, but the relative humidity of the remaining air inside rises, and may even reach the dew point causing more condensation in the safe. The next morning the whole process repeats again. Every day there is more water inside the safe.
Depending on the environment around the safe a heating element may not be enough to prevent excess moisture inside. The best solution short of making the safe airtight and purged with dry air or nitrogen is to either reduce the temperature changes or the humidity outside the safe.