A sneak gives secrets away.
Those of us who are old to have read 'Jennings' and 'Billy Bunter' stories will have encountered the word 'sneak' many times.
e.g.
‘For goodness sake, keep this dark, Bunter!’ exclaimed Wharton.
‘Eh?’ Billy Bunter blinked at him. ‘Think I’d tell Quelch? I’m not a sneak, I hope.'
[Billy Bunter: The Hiker, Frank Richards]