Some kind of "consent to be owled" thing? Sirius could be in a strict "don't owl me" setting (enchanted himself?), except for individual, pre-arranged owls, of whom Hewig was one?
The more consistent one was: Hedwig couldn't tell Harry where Sirius was, so does it matter that all enchanted (actually, I'd go for a special magical breed of owls myself) owls could find him, if nobody could find out where they found him? Yes, they could put some kind of tracker on the owl, but, given wizardly predelictions for secret locations (exactly why was the Black family home so hard to find?), I'd imagine that a resistance to such tracking devices was intrisic to the owl system. Animals in the Potterverse, after all, can have their own powerful, humans-no-go magic. All of which is better evidence that the trick is in the owls' own magic, not wizard magic put on them.
Also, I am now hugely amused at comtemplating the notion of wizard spam. Sirius, hidden out halfway across the world, and still getting offers for credit cards from Gringott's.
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The more consistent one was: Hedwig couldn't tell Harry where Sirius was, so does it matter that all enchanted (actually, I'd go for a special magical breed of owls myself) owls could find him, if nobody could find out where they found him? Yes, they could put some kind of tracker on the owl, but, given wizardly predelictions for secret locations (exactly why was the Black family home so hard to find?), I'd imagine that a resistance to such tracking devices was intrisic to the owl system. Animals in the Potterverse, after all, can have their own powerful, humans-no-go magic. All of which is better evidence that the trick is in the owls' own magic, not wizard magic put on them.
Also, I am now hugely amused at comtemplating the notion of wizard spam. Sirius, hidden out halfway across the world, and still getting offers for credit cards from Gringott's.