catch someone with their hand in the cookie jar
catch (one) with (one's) hand in the cookie jar
To catch one in the act or attempt of some wrongdoing, especially involving bribery or the illicit exploitation of one's position of power or authority. Likened to a child literally being caught trying to steal cookies. The authorities haven been trying to catch that senator with his hand in the cookie jar for years—he's long been accused of insider trading.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
catch someone with their hand in the cookie jarAMERICAN
If you catch someone with their hand in the cookie jar, you find them doing something wrong, especially stealing. The banker was caught with his hand in the cookie jar. So Harry caught you with your hand in the cookie jar? What happened next? Note: You can also talk about a person with their hand in the cookie jar or say that they have their hand in the cookie jar. Among those with their hand in the cookie jar was, it is alleged, the director of the program. Note: The usual British expression is have your hand in the till.
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012