Sneakoscope going off at beginning of GOF?

[christine breen]

No: HPFGUIDX 29831

This has been puzzling me for some time now.  Why at the beginning of
GOF, when they are on the Hogwarts Express in the last cabin with  Lupin,
why does the sneakoscope start going off?   I'm assuming that it went off
in regards to Scabbers being really Pettigrew, but that is not very clear
in the text.  What do you all think?
Christine