We know nothing about Maude’s past. In one scene, however, Harold holds her hand. and she pulls it away. But the camera zooms in and, for a moment, lingers on her wrist, and we can see a number tattooed on her skin. Maude doesn’t notice but Harold is visibly shocked.
It is a tiny glimpse into her past , which is never mentioned explicitly nor alluded to again. She probably was a prisoner in a Nazi concentration camp, where she came close to death so many times. That’s why she appreciates life the way that she does: now her whole character makes sense.
It happens one day, at sunset, on the beach, when Harold and Maude are talking and looking out to sea, and she is about to say:
“Dreyfus once wrote from Devil’s Island that he would see the most glorious birds. Many years later in Brittany, he realized they had only been sea gulls… “
She smiles at Harold and then goes on: “To me they will always be glorious birds”.