The JC has to believe that Dooku is a Sith. After it was discovered that Maul was indeed a Sith the Jedi knew there was another one out there, they just weren't sure if it was the master of apprentice. Surely they must think Dooku is a Sith, its just way to big of a coincedence for him to leave the Jedi Order around the same time the Sith come back.
Obvious as it may seem to us, JO, they clearly showed that they were not treating Dooku as a Sith.
Remember in any case that Dooku only became a Sith AFTER TPM; this is a twist that the Jedi almost certainly did not suspect and so the suspicions of him being a Sith are very low, doubly so after his 'reveal' to Obi-Wan.
As someone they would talk of as "Ok, we've found the other Sith!" Not the rather feeble "Do you think he was telling the truth about the Senate?" conversation we got.
All they know is that he is on the Dark Side; they haven't the remotest proof of Sith involvment. Yoda didn't day "Know he should, after all, a Sith is he!"; all he said was that deceit and deception is his way now.