For context, the full sentence I am trying to understand is "Su vida de ermitaño era un enigma, quizá el de qué secretos ocultaba, cómo podía haber vivido seis décadas sin desafíos notables."
I understand most of the sentence - my rough translation is "His life as a hermit was an engima, ..., how he had been able to live six decades without notable challenges." It's the middle part that is confusing me. "Maybe what secrets were hidden/he hid" is the best I can come up with, but that doesn't seem to make much sense. How would this segment translate into English?