Green’s Dictionary of Slang

scarecrow n.

[? his visibility resembles that of a scarecrow standing in a field]

(UK Und.) a pickpocket’s young assistant, who is disposable and can be turned over to the police.

[UK]J. Greenwood Little Ragamuffin 343: The scarecrow is the boy who has served him [i.e. a pickpocket] until he is well-known to the police and is so closely watched that he may as well stay at home as go out.