Five police documents - one of them highly sensitive - have been found in a dog waste bin in Happy Valley.
Police received a call on Thursday saying the documents had been put in an envelope inside the bin on Clementi Road.
The Sing Tao Daily also reported that it had received a similar call.
Officers rushed to the scene and found the five documents. One of them was a security assessment report of a control room at the police headquarters in Wan Chai.
The document, dated last year, bore the heading "secret" and the name of Acting Superintendent Adrian Tang Chi-ming. A police source said officers suspected an insider who wanted to frame Tang, who is due for promotion.
The culprit could have printed out the documents and cut and pasted Tang's name to frame him, the source said.
The report detailed the security at the entrance to the control room, the number of CCTVs and the locations they monitored.
The remaining four documents were general administrative paperwork.
All the documents were copies, and the original security report was still in a safe at the Wan Chai headquarters.
A police spokesman said the force was very concerned about the case. He said police had taken measures to ensure security had not been compromised.
Previously, police documents, including witness statements and training material, have been uploaded onto a file-sharing network. The Hong Kong Island regional crime unit is handling the latest case.