Snape and Neri's old theory.

[houyhnhnm102]

No: HPFGUIDX 137424

Ladyljd:
> Perhaps, but it could be the beginning of discusion of why Snape was 
> arrogant enough to believe that Potter would just trust him to take 
> care of the problem.  
> 
> He could also have trusted Potter to handle himself with Umbridge in 
> the Forbidden Forest while he dealt with the Sirius problem.

houyhnhnm:

But Snape has no reason to believe there really *is* a Sirius problem.
 He has known since the occlumency lessons that Voldemort is accessing
Harry's mind.

"And he might make me try and do things?" asked Harry.  "Sir," he
added hurriedly.

"He might," said Snape, sounding cold and unconcerned. (OotP, AE, p. 533)

He also has five years experience of Harry's tendency to take matters
into his own hands, but how could Harry possibly get to London with no
broom, no ability yet to apparate.  Even Harry would not have thought
of using thestrals if Luna hadn't suggested it.  Snape has no reason
to think there is a problem until enough time has passed with no one
returning from the forest.