How would Voldemort find out?

[rosered2318]

No: HPFGUIDX 133346

"I'm sorry Harry, I should have said that Voldemort would not want 
to immediately kill the person who reached this island.  He would 
want to keep them alive long enough to find out how they managed to 
penetrate so far through his defenses and, most importantly of all, 
why they were so intent upon emptying the basin.  Do not forget that 
Lord Voldemort believes that he, alone, knows about his Horcruxs." (HBP, Ch 26, Scholastic p569)


This quote strikes me.  After the second read-thru of HBP, my 
secondary reaction was "How would Voldemort find out about the 
intruders and their purpose?"  As Voldemort did not show up to AK 
Harry and DD as they were emptying the basin, I am assuming there 
was not a magical alarm associated with entering the cave or 
drinking the potion and if it was associated with the awakening of 
the Inferi, that would not allow Voldemort enough time to get to the 
cave.

But, as often with JKR's work, the surprise does not come from 
the "who" or the "how" but the effect that it has or the 
circumstances around which it occurs <coughDD'sdeathcough>.  

Some problems I forsee with thinking of this phrase as suspect start 
with the ring and end with how Voldemort would be notified of the 
intrusion.  If Voldemort has set a magical recording device around 
this Horcrux, then was there one around the ring?  If not, why not?  
If so, how would Voldemort know that there was something to see?  It 
would appear as if there was no "alarm" activated.  But this phrase 
allows us an insight into Voldemort's mind that makes a little too 
much sense for me to just pass it off.

The interesting side note (pun intended) - our mysterious Mr. R.A.B 
(whom I am assuming is Regulus Black) still has his note in the 
basin.  This means that Voldy found out about his treachery some 
other way and did not bother to retrieve the phony or perhaps left 
it there out of spite for the next in line.  Perhaps he has placed 
the real locket somewhere else or it was destroyed.  Another 
incidental side note - Regulus must have been killed by Voldy 
personally if DD was correct about Voldy not wanting anyone else to 
know about the Horcruxs' (even Lucius didn't know about the Diary's 
true purpose until afterwards)

Based on DD's quote, I am going to assume that Voldemort knows Harry 
is now looking for his Horcrux.  The question I now ask is how is 
this going to affect Harry's ability to find and destroy them.

Rose