Poison

[Crystal Williams]

No: HPFGUIDX 146809

    Andie:
  <snip>
 Draco tells DD, "I got the idea of poisoning the mead
from the Mudblood Granger as well.  I heard her talking in the library
about Filch not recognizing potions." pg 589 US

Pg 307, Harry & Hermione are in the library:  "--so it would be down to 
Filch to realize it wasn't a cough potion, and he's not a very good
wizard, I doubt he can tell on potion from --"
Hermione stopped dead:  Harry had heard it too.  somebody had moved 
close behind them among the dark bookshelves.  they waited, and a 
moment later the vulturelike countenance of Madam Pince appeared...

Who was actually listening in?  Madam Pince was behind them listening.
Draco isn't mentioned.  Not that that matters but H & H were 
whispering.  They should have noticed if anyone else was around.

How did Draco overhear their conversation?
     
  Crystal: 
   
I'm thinking that Draco was probably who they heard but Madam 
Pince was there also. He was probably behind another bookshelf or 
something and it's not as if they checked really good to see if 
anyone was listening or not. Draco could have left before anyone 
noticed he was there. Either that he could have gotten his hands 
on some Extendable Ears... which to me is doubtful but is a 
possibility.