"...I had to disable my Sneakoscope because it wouldn't stop whistling."
--Impostor Moody (GF20)
The Sneakoscope is a device which looks something like a gyroscope and which gives off a whistling sound when someone untrustworthy is around.
- Harry was given a small one by Ron (who bought it in Egypt while on holiday). It was a cheap souvenir so the boys ignored its warnings (PA1)
- The fake Moody had a large, extra-sensitive one in his office (GF20)
- Hermione gave Harry one for his seventeenth birthday (DH7)
The Sneakoscope went off when Harry and Ron were in the train carriage. There didn't seem to be a reason for this so the boys dismissed it as a defect, but it could've been because of Scabbers. Scabbers was with them in the train carriage and the sneakoscope could've detected that he was really Pettigrew, which would make sense because Pettigrew is an untrustworthy person.