Harry's ability to detect traces of magic

[gelite67]

No: HPFGUIDX 151409

I've noticed two places in the series where Harry has possibly 
noticed the "presence" of or traces of magic:

First, in Ollivanders:  "...the back of his [Harry's] neck prickled.  
The very dust and silence in here seemed to tingle with some secret 
magic."  U.S. Edition, softcover at 82.

In HPB, at the cave:  "Harry could not tell whether the shivers he 
was experiencing were due to his spine-deep coldness or to the same 
awareness of enchantments."  U.S Edition, Hardcover at 557-58.

I'm wondering if anyone can think of any more instances?

I'm also wondering if magic leave traces why/if traces of magic can 
be sensed at Hogwarts, or if it is more easily detectible where magic 
otherwise shouldn't be practiced???

Finally, I'm wondering if detecting traces of magic something that 
can be learned?  DD sure didn't give much guidance on the subject, 
which seems very odd if Harry is going to need to rely on being able 
to detect traces of magic in order to find the Horcruxes.

Angie