Understood. Just a watchout for others

Storing any gun unattended for a long period of time is risky. Too bad your friend didn't have someone to check on those guns. I've been storing guns in sleeves for well over 10 years without any negative issues, but again, I pull them out, check them over, and wipe them down regularly. But I'll be extra vigilant about the stocks after reading your post -- thanks!

His whole household belongings were in a commercial storage facility. Perhaps that had something to do with it. Temperature or humidity fluctuations?

Silicone sleeves can be problematic. VCI sleeves are altogether a different thing. They are designed for storing guns long term in humid environments.