Police have set up a working group led by a deputy commissioner to find ways to better handle material carrying personal data after a series of privacy leaks.
The latest incident happened yesterday when a constable left 18 notebooks on a bus.
Earlier this month two fixed-penalty traffic tickets were lost separately.
In September, 210 documents including witness statements, internal memos and letters were found leaked through internet file sharing software Foxy.
And in August, cross-boundary area permits belonging to 79 pupils at seven kindergartens were lost in Sheung Shui police station.
Privacy commissioner Allan Chiang Yam-wang confirmed last night he had launched an investigation into the Foxy leak.
Police appealed for the public's help yesterday in tracing the 18 notebooks. The officer, attached to the marine police regional crime unit, was on a Kowloon Motor Bus vehicle on route No23 from Sau Mau Ping to Kwun Tong at about 7.30am.
He was on his way to the marine police headquarters in Sai Wan Ho to return the used notebooks, police said.
The notebooks, in a black paper bag, were left on the bus when the constable got off at Kwun Tong. By the time he realised the books were missing, the bus had left.
He reported the incident to the police, who contacted KMB and conducted searches in various locations, but to no avail.
A senior police officer said the blue notebooks - with the constable's name and police unit printed on them - contained records of what the constable had done, where he went and who he interviewed.
"We are looking into what information was in the notebooks and what cases the officer was involved in," the officer said.
Anyone who found the notebooks is asked to call 999 or contact officers at 2803 6240. The force is also investigating whether the officer was negligent.