I though Switzerland's defense was more based around being mountainous and having explosives planted in critical tunnels/bridges and having a large percentage of the population armed and trained for national self defense. Basically in a ground invasion would be deadly and slow with no mobility and resistance everywhere.

Besides that it's hard to imagine which foreign power would have incentive and power projection to even try it.

The way I've understood it is that none of their defense is hidden, it uses the natural and societal benefits. Most of that could be gleaned by anyone with access to satellite images and wikipedia.

That might all be outdated info though, it'd be interesting to see where I'm wrong though!