Snape & Sirius

[lipglossusa]

No: HPFGUIDX 31455

I have a question about something that happens in PoA 18, when Snape 
confronts them all in the Shrieking Shack.  No one realizes he's 
there until he decloaks- and in the confusion that ensues no one 
stops to wonder how long he'd actually been there, listening under 
the invisibility cloak.
I looked back in the chapter and found the part where the door opens 
by itself, or so they think, but this is most likely where Snape 
slipped in, invisible.  Ron remarks that the house is haunted.  Lupin 
tells him that it's not, that the ghost story was a coverup, etc etc.
Then, Lupin goes on to tell H,R,&H about his years at Hogwarts-- and 
that James, Sirius, and Peter became Animagi.  He also specifically 
states that MWPP (himself included) were the ones who wrote the 
Marauder's Map.  Snape hears all of this.

So my question is-- Why hasn't Snape told anyone that Sirius is an 
Animagus?  Why hasn't he tried to confiscate the Marauder's Map 
since?  And why, at the end of GoF in the infirmiry, does he express 
surprise when the black dog turns into Sirius, since he already 
really knows that Sirius is an Animgas?

I think I can answer the latter one myself-- Remus Lupin never 
specified what sort of animal they all were-- he just says that 
Sirius and James transformed themselves into animals that were large 
enough to keep him controlled in his werewolf form.  So, I guess 
Snape didn't really know what kind of animal Sirius becomes until he 
sees him transform.

But I still don't see why he hasn't told the Ministry or anyone else 
that Sirius is an Animagus, because he most definitely knows.  The 
door opening "by itself" is clearly the point where he enters the 
room in the Shrieking Shack and hears Lupin's story.  Then in GoF, 
when Sirius sneaks back to see Harry, hiding in the mountains and 
traveling as a dog-- Sirus even says at one point, to reassure Harry, 
that he wasn't in terrible danger of being caught by the Ministry 
because they still don't know he's an Animagus.  I refuse to believe 
that Snape merely "forgot" this information-- although you could make 
a case arguing that Snape's memory was damaged after being stunned. 
But he seemed to remember everything else clearly enough when he 
tells Fudge his version of the events.  I would think Snape would be 
most eager for Sirius to be re-captured, and the fact he still has 
not shared this vital information with the MoM is very strange.