I have one of such fortifications, called Toblerone line, running around our place in canton Vaud. The whole line is maybe 15km long from Geneva lake up the Jura mountains, made up from concrete spikes (even raisable in the middle of roads), blowable bridges and around 15 tiny concrete fortresses (fortinettes), all in plain sight, mostly visible on google maps/street view.

All built around 1930s. There is popular hiking trail along all this, since nature and forests around it are pretty and well maintained. No secret really. Same for many other old stuff. New stuff should be hidden on bases/remote places.

Btw checked our building and surrounding ones and its pretty precise but not up to date with 2024 finished construction.