Sometimes you have to look closely for them, and I imagine it was even harder when they were camouflaged.
This overlooks the small harbor at Charlestown. See the bunker to the left of the house?
A public swimming beach, the bunker is about 1/3 of the way up the slope, even with the right edge of the house above.
Now being used as storage for a kayak rental business.
When I saw all these bunkers my first thought was that many of them must have been completed after any real threat of invasion had passed. The time interval from the Fall of France to the end of the Battle of Britain was a matter of months really. Even if you figure that some threat of invasion remained in theory until Hitler invaded Russia, that's only a year. So many military schemes these days have a long planning stage and are obsolete when finished.
But I underestimated the Brits. It seems that most of these bunkers were actually completed in 1940 or 41. Bunker construction went on at a furious pace. One source I encountered said that at peak construction in the summer of 1940, a bunker was finished every 20 minutes! We forget how much can be accomplished under existential threat.
After a while though, watching the seas for an invasion that would never arrive must have become tedious. But it still had to be done. The British were mindful of the need to keep the Germans from landing spies and saboteurs. That of course being exactly the sort of thing that the British did all the time and the Germans almost never. By 1943/44 that need was actually rising, Cornwall was a staging area for the Normany invasion and any slip in the secrecy that surrounded Operation Overlord would have cost many thousands of lives.
It must have been a lonely vigil. High up in the cliffs west of Falmouth we found a monument to this diligence:
"Dad's Army", they also serve who only stand and wait.