Schubert, His Opus 11, and the Visitor...

Many years ago, while still in Germany and immersed in advanced studies, I was also performing for a group called Amerika Haus, in many places; mostly throughout Germany. And as I had mentioned in an earlier blog, I was given use of a wonderful Bechstein piano to prepare my recitals,  in Frankfurt, at the Palmengarten, a beautiful building with gardens and palm trees right in the middle of the structure.
I recall one day very vividly; while working on Tchaikovsky's  "Dumka," a young man came through the room (people were free to come into this room, as it was a public building. It really was beneficial to me, as very rarely does one have the experience of practicing before a constant, roaming audience, and this constant 'intrusion' served  to enhance my concentration).
At any rate, this young man approached the piano, armed with  manuscript. He listened for a few moments, and when I paused, he asked me if I would accompany him for just a short time. I was mildly surprised, as he had appeared from nowhere. He then told me that he was a student of voice, and had been given a copy of  Schubert's   "Die Nachtigall"  earlier that day, and would like to try it out.
This was a rather strange, unanticipated moment for me, as the request was so sudden and, I thought, rather unique.
And so I decided, after a few  seconds of  "what should I do?,"  to take the score from him, look it over briefly,  and say to him something like "let's try it."
He sang really well, and I perceived that he had been  strongly  familiar with the composition long before he had received it.
Part of the way through it, he paused, thanking me for my  accompanying him. He then asked if he could sit for a few minutes to hear me work on the Thaikovsky.
He then shook my hand, thanking me once again and departed.
He never returned - I never saw him again.
I  think about this rather singular encounter from time to time;  for me, an unanswered question...