Directly from the manufacturers (was PoA Chapter 14 Summary)

[Amy Z]

No: HPFGUIDX 20553

Pippin wrote:

>  He confronts Lupin with the map hoping to surprise him
> into a guilty admission that he gave it to Harry.  Whatever excuse
> Lupin comes up with (and it's not a great one, IIRC), Snape isn't
> going to buy it.  So Snape doesn't need to pursue the matter in 
front
> of Harry and Ron.  He's got what he wants: Harry has a map once 
owned
> by James and gang; Lupin is the only gang member on the premises;
> Harry now has the map; ergo Lupin gave it to him; 

Snape can't be sure on this point.  I agree he's trying to surprise 
him into admitting he gave it to Harry--but failing a confession from 
Lupin, Snape has to consider the possibility that James left it to him 
and Lupin didn't even know he had it.  Maybe that's why Snape doesn't 
say anything to Dumbledore--although he really ought to, not to rat on 
Lupin but to make sure it doesn't get back into Harry's hands.  
OTOH, maybe Lupin's "I'll take this back" is a clear enough message to 
Snape:  "It's mine, I know you know it's mine, and I'm going to keep 
it--I won't let Harry have it."  

Still, the other reason Dumbledore should be told about it is that it 
would be very useful in watching for Black.

Amy Z

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 Grenouille: I cannot go with you to the
   market today, Crapaud.
 Crapaud:  But Grenouille, I cannot carry
   the cow alone.
             -Quidditch Through the Ages
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