I listen to audio books while I drive. I'm saying that so you will understand why I have never see this word written. This discussion is about Harry Potter and the Order of the Phoenix.
The first time I heard this word someone was opening a letter and after it came from the envelope the statement was, "When the letter was unfeld ..." Â I could not believe I heard this right, but I did not replay it since I was driving. I assumed that either I heard it wrong or it was another British way of talking and I soon forgot it.
A few days later, in a similar situation, I heard the word again. This time it was convenient to replay the selection so I did and found that I had heard right. They are using the word unfeld as a past tense of unfold. When I got home I tried to find it in some dictionary on the internet but I could not find this word used or mentioned in any of the results of Google.
So, I hope someone can shed light here. Is "unfeld" a legitimate word anywhere in the world? I have only heard it twice and both times it had the prefix. So if "unfeld" is a word, is "feld" also a word? I can see the logic, being that it is similar to "hold" and "held". Does anyone know?
After I posted above, I did a different search and found 3 or 4 sites saying that "feld" is not the past tense of "fold". I tend to agree, but why is J.K. Rowling using it?
google book finds some. I think that's what it is, past and past participle of unfold, same as unfolded.
Without randomly digging through the book, I will suggest that another  possibility is that you're hearing a British pronunciation of  unfurled.  Pure guesswork.
Tromboniator, I love your guess, but how can a letter be furled inside a letter? An extension of meaning?