Yeah, moisture can still seep through the crevices of the box if it's kept in areas that are wet and/or humid. This is likelier to happen in tropical countries.
I bought an MISB Encore G1 Meister (Jazz) from an eBay seller in Hong Kong about 7 years ago and never opened it. Last year I noticed some white spots (mould growth) appearing on the inside of the plastic window. Also, looking through the plastic window, I noticed the car mode's axles (center of the wheels) were rusted. I ended up opening the box, cleaned off the rust and mould with WD-40.
I think it's a good idea to store MISB toys in a ziplock bag (you can find a variety of sizes at the supermarket) with a packet of silica gel to absorb any existing moisture.
Problem I have now with my MISB Encore Soundwave is that it's packed away in a box behind a shelf (hard to get to), and I got it before the Encore Jazz I mentioned earlier. So any mould and rust inside would have probably accumulated by now![]()