Sneakoscope question

[ladyramkin2000]

No: HPFGUIDX 95619

I'm so glad someone has raised this question, as it has been puzzling 
me since first reading The Prisoner of Azkaban.  I first thought it 
was Lupin's presence that set the thing off, but obviously it wasn't 
that.  Nor could it have been Scabbers, as he had been there all the 
time without any reaction from the Sneakoscope.  It could have been 
because Lupin was only pretending to be asleep - I've always thought 
that he wa s fully aware of all the conversations going on around 
him - but that doesn't seem a very convincing explanation.  Has 
anyone got a better idea?

Sylvia