How did Snape see Lupin the night of the Prank?

[Judy <judy@judyshapiro.com>]

No: HPFGUIDX 50747

This question has long intrigued me --
how *did* Snape manage to see Wolf!Lupin?

Steve proposed a Barrier Charm: 
> ... the Shrieking Shack could have
> an EXIT barrier charm, that... blocks any
> attempts to exit the house..... 
> if James hadn't
> stopped Snape, he could have entered the house. 
> Once he entered, once
> he went past the barrier charm, Snape would 
> have been trapped in the
> house with Wolf!Lupin with no means of escape.

This is a very clever theory, but I see a flaw.  Lupin says that James
put himself in great danger to save Snape.  If there were an "exit
barrier" charm, with no way at all to get out of the Shack during the
full moon, then James would have been in no danger; Wolf!Lupin would
have had no way to reach him.  Since James didn't go into the Shack
that night, he would have been safe the whole time.

Still, Steve, I think you are right that we should look for a magical
solution.  I like the idea of an invisible barrier.  Instead of the
one-way barrier that you describe, how about a regular (two-way)
invisible barrier, that could be opened with an easy spell such as
"Alohamora"?  We can assume that Wolf!Lupin would be unable to cast
spells, and therefore the magic barrier would have contained him
effectively. Madame Pomfrey (or whoever) could put up the barrier in
the evening. Perhaps even Lupin himself would be the one to do it,
while still in human form. In the morning, the barrier would be
removed, either by Human!Lupin or someone else. 

We also can assume that werewolves are powerfully magical and hard for
even wizards to defend against. So, Snape's life was in danger because
he might have opened the invisible barrier without knowing what was
behind it. (Perhaps while Wolf!Lupin was upstairs in the Shack.) 
James' life was in danger because if Snape opened the barrier,
Wolf!Lupin could have gotten into the tunnel and killed him, too. 

Perhaps James and Snape argued in the tunnel, near the Shack.  The
sound of human voices might have attracted Wolf!Lupin to the barrier,
where Snape could see him.

At the time that Sirius drags Ron into the Shack, the barrier wouldn't
be there at all, since it would only have been put up when Lupin was
confined there.  

-- Judy