I think the most practical solution is a small, unassuming military installation. Nothing that would attract attention, more like a side branch of some logistics chain or a third rate weapon storage site. There are thousands of such places, they blend in well with the neighbourhood and they don't look weird when actually guarded.
I've seen crumbling factories and they're just not a good place to hide anything, as they're teeming with all sorts of people, like metal thieves, souvenir collectors, homeless, drunkards, architecture students or adventurous teenagers. Not a good place to hide anything secret.