I keep SLR my film on a dry box, it has been there for about 3 months now. I always keep the RH level from 37-48. I took out the lens from the mount and saw a small brittle rubber lying down at the bottom of the mirror. I fired a few shots and it's still working. Am I storing my camera the wrong way?
The mirror dampening material, just like that of the light seals, is made of foamy rubber which degrades over time and becomes brittle. Unfortunately you cannot prevent this from happening.
This isn't super catastrophic as this material is fairly and cheap to replace. Since you've noticed the material falling apart, I suggest you to replace the seals and mirror dampener.