Who is the snitch in Tantei Gakuen Q?
The so-called mole is the subject of much debate between the manga and anime canons... 

In the manga,


   The secretary, Shino Katagiri... or rather the Pluto/Meiousei agent masquerading as her. The real Shino had was kidnapped before Dan Morihiko. Luckily they find and rescue her. It's assume that she was kidnapped later on.


Additionally:


   Ryu was originally sent by Pluto/Meiousei as a spy/envoy (due to his status as the their of the organization) inside DDS, particularly to keep an eye on Dan Morihiko, but he later defects from Pluto/Meiousei. 


In the anime, 


   Sir Anubis is the one that initiated the kidnapping of Dan Morihiko. He apparently hat Ryu under surveillance, after tries to leave Pluto/Meiousei, tries to use Dan-sensei as a bargaining chip to get Ryu to return.