How does McGonagall know?

[oz_widgeon]

No: HPFGUIDX 32625

Okay, I lied, I'll go back to the sofa in a minute and continue being 
miserable.  I have another question that popped up while re-re-re-re-
reading PoA.
First, let me put my quotes in:

Chapter 13 Gryffindor Versus Ravenclaw:

[Gryf beat Ravenclaw at Quiddich and are in the common room 
celebrating]
'The Gryffindor party ended only when Professor McGonagall turned up 
in her tartan dressing gown and hair net at one in the morning to 
insist they all go to bed.'  (PoA 265 US)

[They have all gone to bed.  Ron wakes up screaming that Sirius Black 
tried to attack him, the whole house wakes and noise begins again]
'"Now really, enough's enough!"  Professor McGonagall was back.  She 
slammed the portrait behind her as she entered the common room and 
stared furiously around.' (PoA 267 US)

Now, we know from CoS I believe, that Harry has only seen McGonagall 
in the common room a few times, so she's not a regular visitor.  
Twice in one night seems a bit of an event.  My question is, how does 
she KNOW they're up?  I get the impression that the walls are quite 
thick, and muffle sound.  If they didn't, wouldn't it be quite easy 
to figure out where the other common rooms are?  (Follow the noise).  
So, does McG live somewhere off the common room, though perhaps with 
a private entrance that she can hear things in the common room, or 
does she have some type of magical device that allows her to keep 
tabs on them?  The latter theory bothers me, as she then would know 
everything the trio plots and would also know about Sirius in the 
fire in GoF.  Any thoughts?

Slon (who is REALLY back to the sofa now as her head seriously hurts)