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Dumbledore had an easy time detecting the leftover magic in the cave near the end of the sixth book. In the seventh, the trio cast a heck of a lot of charms in order to hide their presence. However, if witches/wizards can detect traces of magic, then why does casting a heck of a lot of spells help you hide from them?

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    Not sure that what Dumbledore can do is representative of what is 'easy' for most witches & wizards, and even then he had worked out where to look by other means before he specifically looked for traces of magic.
    – Michael
    Nov 20, 2021 at 15:21
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    In fact, I would argue that Dumbledore doing that is one of the VERY few wizards capable of doing so in the wizarding world. I bet there are no more than can be counted on one hand. And he was in a place specifically where he was seeking such magic. Nov 22, 2021 at 19:59

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In the case of Harry, Hermione and Ron casting spells in book 7 to hide themselves, it all falls on the idea of 'leaves' traces. When a spell is performed and finished the residual magic left behind can be detected by powerful wizards/witches. But while the spells are in action the magic is being used, especially in the case of the trio casting protection and concealment spells. The magic is on the inside where the trio are, whereas from outside the bubble nothing can be seen or detected. Once the spells have run their course and faded away they leave a trace behind. A reason why the trio never stayed in the same place twice.

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    Sources for this?
    – J. Mini
    Nov 20, 2021 at 17:59
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    Elements of this are probably right, but I'm not seeing a lick of evidence, either here or in the books.
    – Valorum
    Nov 20, 2021 at 18:55
  • @Valorum the last two sentences are a but speculative, yes, but, "The magic is on the inside where the trio are, whereas from outside the bubble nothing can be seen or detected," is accurate throughout the majority of Deathly Hallows, particularly when the Trio is caught by the snatchers. Although not included in this answer, there is plenty of evidence in the books.
    – ava
    Nov 23, 2021 at 22:40
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There is also the difference between knowing that someone cast some magic in the area and those pesky children you're looking for cast magic in the area.

As pointed out in this question How did Dumbledore detect the "traces of magic" in the Cave? Dumbledore "knows" Tom's style. Whether that is an actual "fingerprint" that identifies Tom as the magic user or simply that Dumbledore knows what kind of magic/spells Tom would likely have casted is unclear. But it seems having some knowledge of the magic user is helpful.

So it's likely that a random magic user looking for someone they may only know the name of, isn't going to be able to find out much more than simply that some magic was used in the area, rather than the more precise form that some magic was used in the area by the person they are looking for.

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  • You might also want to note that Tom's magic is evidently powerful enough that traces of it have embedded themselves in the walls whereas Hermione's wards are specifically intended to hide users from detection.
    – Valorum
    Nov 22, 2021 at 11:31

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