A safe is not really a safe place for guns!
I've had humidity problems in my safe on more than one occasion. Years ago my safe was in a spare room where I had closed off the heater vents - bad idea! I turned the heat back on and started using
Dri-Z Air. The problem went away. The I moved to a different house and the safe went into an un-finished and un-heated part of the basement. Even though I added a heater (Goldenrod type) it was not enough. I added more heat (a total of 18 watts) and still use Dri-Z-Air. No more problems (so far).
Just because you don't see actual condensation doesn't mean there isn't enough humidity to cause rust. I experienced the same rust as you did on a stainless Ruger P95DC with no detectable condensation.
Oh - by the way - Stainless is not. Stainless rusts and corrodes just like any other steel - just not as much or as fast.