A nuclear bomb is a slightly different animal than a chemical bomb.
OK. So where would you keep one or two or three if you had it? An underground bunker under six meters of concrete sounds like a pretty good place to me. The biological warheads were left lying around in dumps, littering the landscape by comparison, and the missiles in the photographs provided by Diogenes #32 were buried in sand.
There must be something rather special about what was found, if the story is true, for the weapons (missiles or bombs,) to be so carefully concealed.