Tree felling work uncovers WW2 underground bunker near Moffat
bbc.com
bbc.com
It was so heavily dependent on local knowledge that it was only found because someone who used to play in it as a kid went looking for it.
That's how you keep a secret.
https://www.amusingplanet.com/2017/06/operation-tracer-secre...
It was not found until recently. They also found one of secret operatives alive (doctor).
https://www.google.fi/amp/s/www.warhistoryonline.com/world-w...
Once one or two of them are museums and the known survivors were documented, there's not much archeological or historical value in hunting them all down.