Let's hear it for good old Snapey!

[KathyK]

No: HPFGUIDX 87923

Ces wrote:
 
>One possibility is that once Snape saw Black's name on the Map, he 
didn't look any further and just ran for the shack.<

KathyK:

Snape didn't see Sirius' name on the map, he saw Lupin's.  
Specifically, he saw Lupin's dot heading off the map to the 
Shrieking Shack.  Sirius was already off the map as he was already 
in the Shack at the time Snape looked at the map.  Peter was also in 
the shack and thus not appearing on the map when Snape found it.

As Snape says, he suspected Lupin was helping Sirius and when he saw 
Lupin heading off to the Shrieking Shack, he believed his 
suspicions were confirmed.  Since Lupin chose not to share important 
information about Sirius with Dumbledore, Snape was right in a sense.
So once Snape saw Lupin rushing down the passage, he went after 
them, just knowing he'd catch Lupin helping Sirius.

KathyK