A shelter at Atlantic GC comes to mind.
It's cut into a dune/mound, completely hidden from play and appearing as a natural feature on the golf course.
From the non-playing side, it has an full entance and is easily accessible.
The only negative is that it's so well incorporated into the architecture that an unaccompanied guest would never know it's there.
What other courses hide or blend their shelters to the degree that they're indistinguishable from the rest of the golf course ?