In the anime Detective Academy Q/Tantei Gakuen Q, at one point we learn that there is a snitch in DDS spying for Pluto/Meiosei.
At first we are led to believe that it is Hongou Sensei, but later Hongou is loyal to DDS when they go to the cruise ship so he probably wasn't the spy.
Katagiri Sensei had been briefly kidnapped and impersonated in the events leading up to the finale, but IIRC she is not being impersonated for very long, ruling her impersonator out as the snitch.
Ryu hates Pluto so can't be the snitch. Unless Pluto had bugged him (there is no confirmation of this in the anime)
So who was the snitch/spy? I'm talking about the period between when Hongou Sensei came in to before Dan was kidnapped.