Two senior Victorian police stopped recording notes in their diaries at the same time it is claimed they first learned Nicola Gobbo was a police snitch.
Records before a royal commission allege Chief Commissioner Graham Ashton and Assistant Commissioner Luke Cornelius learned of her informer status in 2006.
They deny this, arguing they found out in 2007 and 2009 respectively but have no records of their own to back it up.
Mr Ashton was a top police corruption investigator at the independent Officer of Police Integrity at the time, while Mr Cornelius was in the force's internal ethical standards department.
The two stopped making notes in their diaries in 2006.
Mr Cornelius on Thursday rejected notes made by colleagues that year indicating he was told of Ms Gobbo's informer status, saying he had no recollection of being told.
When a second diary from another colleague backed up the first, Mr Cornelius described it as "Chinese whispers again".
Counsel assisting Megan Tittensor questioned whether his memory might be faulty.
"I think it quite unlikely because if I'd been informed of these matters it would have certainly triggered some quite significant questions in my mind," he said.
Ms Tittensor then said: "It seems to be it didn't trigger those things with (former chief commissioner Simon) Overland or Ashton or anyone else for that matter."
Mr Cornelius replied: "Well, that may be the case."
He defended his lack of diary notes, saying it was for the sake of information security because he was involved in investigations classified as highly protected.
Mr Ashton gave a similar explanation in his evidence over three days this week, explaining he stopped making notes because he did not believe they were properly protected under legislation from being subpoenaed.
He maintained he did everything he had to in order to stop the scandal.
"I've taken the actions that were required of me at all times ... I reported it to my superior officers," he said outside the inquiry on Wednesday.
"The action I took was appropriate with the level of knowledge I had."
More top police are to testify before the inquiry wraps up for the year, with retired deputy commissioner Sir Ken Jones to give evidence on Friday.
He will be followed by Mr Overland and fellow former chief commissioner Christine Nixon next week.
Australian Associated Press