We're introduced to the Deluminator several times throughout the books, however it is always presented as a magical device which can remove and replace the light in a room or area. How does that match up to the way in which it suddenly was able to lead Ron back to Harry and Hermione? I can't see how the two functions are related. Is this just a new feature of the Deluminator that was never foreshadowed in any way?
I don't believe there was any specific foretelling, but there was a running 'light in the dark' metaphor J.K. Rowling built throughout the series, and I think the Deluminator became the physical manifestation of that metaphor. I do not, however, think there has ever been any direct or predictable link between the metaphor and the Deluminator itself. It's entirely possible it grew into this purpose over the course of the series since we do see a name change for the device; 'Put-outer' becoming 'Deluminator'
The quote Izkata identified from the Prisoner of Azkaban movie is most likely a re-coloring of that scene with J.K's involvement in the screenplay. Though the movie was released ~3 years before the completion of Deathly Hallows book, where we see the previously undiscovered use, it could easily have been the intent of the author to use the movie as a vehicle to further deepen the metaphor in anticipation of its further use within the plot.
Potential references that may fill in backstory (more than foretelling) around the Deluminator and metaphor.
- During the bestowing from Dumbledore's will, Scrimgeour calls it a valuable, unique item, certainly of Dumbledore's own design. Implying speculated purpose beyond the simple cantrip of common use.
- After Ron leaves, and before his return, Harry and Hermione discovery Kendra/Ariana's grave in Godric's Hollow where the following quote, presumably by Dumbledore, appears: "Where your treasure is, there will your heart be also."
None of this foreshadows the additional power, but together it can build up to describe it decently. In sad, dark times the Deluminator, a lighter, can illuminate your path and show the way (back) to your heart.
Dumbledore's dark and sad history could certainly have led him to build the Deluminator in such a way as to also guide himself (and others) back to a path out of darkness.
It's all a bit of inference and speculation, but such is the way of metaphors some times.
If you want a bigger stretch ... "Deluminator" could also be read as "De luminator" or "The Luminator" which could have been the author's attempt at tongue-in-cheek foreshadowing within the name change. Implying not just a means by which to extinguish a light, but also shed it.
No, there was no clear foreshadowing of this ability of Deluminator.
Neither in the books before DH (the only use we see is the turning on/off the lights).
Nor in JKR's interviews. The only mention of Deluminator was:
Ron got it because he (Dumbledore) knew that Ron might need a little more guidance than the other two` - from "JKR and the Live Chat, Bloomsbury.com, July 30, 2007"
Nor was it foreshadowed in the scene where the Will is read, for all the Will said was:
"' To Ronald Bilius Weasley, I leave my Deluminator, in the hope that he will remember me when he uses it '
This seems to only be in the movies, but here's a quote from Dumbledore from Prisoner of Azkaban:
The movie was released between Order of the Phoenix and Half-Blood Prince.