Jango Fett was one of(if not the best) bounty hunters in the galaxy at the time, though he was doing one job for the Republic it didn't preclude him from working any jobs that went against them(like attempting to murder a certain senator), Obi-Wan and the Council probably just assumed he was just going from one job to another and that Dooku wouldn't have been told about it. They were also already under the(correct) impression that Sifo Dyas was the one who hired Jango to create the army(and he did, he was just murdered after the fact), since nobody investigated Sifo Dyas' death until towards the end of the clone war they couldn't determine that Dooku was involved in his death, and by the time they did investigate it was to late for them to actually do anything about the Clones, and the council just decided to move on despite the risk. The TCW episode "The Lost One" in Season 6 goes a bit into Sifo-Dyas and his role in the Clone War.