Detecting magic

[Judy]

Kirstini mentioned magic that the Ministry should have detected, but 
didn't: 
> Not to mention the use of (at least) Alohomora, Scourgify, the 
> Packing  Charm(!) and yet another Hover (Locomoter Trunk) by 
> Tonks/other members of the Order at Privet Drive in OoP. Surely in 
> the wake of a breach of the Reasonable Restriction of Underage 
> Sorcery decree, the Ministry sensors would be charged and pointed 
> on young delinquent Potter. 
> You know, scratching around for flints always leaves me feeling 
> slightly sordid.


Yes, I fear this is one of those areas where JKR has painted herself 
into a corner by making magic too strong.  

About the only non-flint explanation I can see is if the ministry 
doesn't have nearly enough resources to detect all magic, and most 
just does random spot-checking to deter underage wizards.  In this 
case, they might have just happened to be checking Privet Drive when 
Dobby did the Hover Charm.  However, there would also have to be some 
way for the Ministry to know about Aunt Marge.  It would be too much 
of a coincidence for the Ministry to also catch this with a random 
spot check. Perhaps some wizard saw her drifting by and notified the 
Ministry?

As for Harry being caught making a Patronus, Umbridge set Harry up in 
the beginning of Book 5, so she could have made sure that the 
Ministry saw everything.

On the related topic of detecting traces of past magic, we've seen 
just one wizard do that - Dumbledore.  It's possible that he is the 
only one who can.  It's also possible that even Dumbledore can only 
detect past magic if there's been a *lot* of it.  He said that the 
cave had "known magic," but  the cave had been exposed to tons of 
magic; think of all the spells on the inferi alone.  True, Dumbledore 
was also able to detect the chain that raised the boat, but this may 
mean just that the chain was an invisible enchanted object, rather 
than Dumbledore needing to detect traces of *past* magic to find it. 

-- Judy