When the pronoun "I" occurs outside of quotes, is it always a narrator's voice? If there are exceptions, I would be grateful for examples.
No, an unquoted I need not always refer to the narrator.
When using free indirect discourse to simulate streams of consciousness, thoughts of characters appear freely in the text without any quotation marks. In such a case, an unquoted I may refer to the character that is thinking (thinking about themselves).
Joyce employs this technique in A Portrait of the Artist as a Young Man and Ulysses.
Consider the following example from the first page of Ulysses.
—I was, Stephen said with energy and growing fear. Out here in the dark with a man I don’t know raving and moaning to himself about shooting a black panther. You saved men from drowning. I’m not a hero, however. If he stays on here I am off.
The first I is in quotes (Joyce uses the mdash for "perverted" commas). That is Stephen talking. The second I, however, is part of free indirect discourse: Stephen thinking. That I is Stephen thinking about himself in his thoughts.
Stephen is not the narrator.
One finds similar unquoted I's for other character's thoughts as well.
In some books, unquoted italicized text is the main character's thoughts.
This occurs primarily in third-person limited (3PL), as most books are written.
In 3PL the narrator is never referring to or talking about themselves; they refer only to characters by name or pronouns.
In 3PL, speech is still quoted:
"I'm not doing that," Mary said.
John winced. "Well, at least think about it."
I already did and it's not happening, John.
But she didn't say that; she rolled her eyes.
"Alright, I'll think about it."
Since the narrator does not ever refer to themselves in 3PL, it is clear to the reader that italicized text with "I", "me", "mine", etc is referring to the POV of the main character for that story segment.
The other answers so far have other types of quotes, that is, we are still meant to interpret the sentence as quoting someone's words or thoughts. But maybe there are cases that refer to the word "I" as an abstract concept. How about these?
That planet's collectivist society had never heard of a singular entity. They had no concept of me, myself, or I.
The ultimate punishment was the reprogrammer. It completely replaced the criminal's dangerous personality with a new one; an I for an I, if you will.
Descartes wrote, "I think, therefore I am". But is the I who thinks necessarily the same entity as the I who exists?
In all these examples, I think one could add quotes and they would work, but I think they're correct as written, too.