Poison

[agdisney]

No: HPFGUIDX 146788

Andie:

Looking through HBP I came upon something that I can't figure out,
maybe everyone else can?

On the tower, 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?