Police are investigating the discovery of snooping devices attached to public computers in two Cheshire libraries.
Staff found the keyloggers, USB devices which record keyboard activity, in the back of two PCs at Wilmslow Library and one at Handforth Library.
The devices are legal but can be used to monitor personal information such as passwords or bank card details.
Cheshire East Council has now checked the rest of its public computers for other devices and increased security.
Staff managed to confiscate two of the devices but the third had been removed before a supervisor got to it.
Councillor Andrew Knowles, cabinet member responsible for health and wellbeing, praised the library staff for their vigilance.
He said: "Keeping library users' personal details and communications private and secure is of paramount importance to Cheshire East Council.
"We take this matter very seriously indeed and are now awaiting police reports on their findings.
"We will take all practical steps necessary to protect people's privacy and personal details and I urge the public to help us by being vigilant and sensible when using public computers at all times.
"If users have any concerns, they should alert our staff immediately."
Experts from Cheshire Police are inspecting the two keyloggers seized for clues about the owners.
Insp Matt Welsted said: "This is an example of how criminals have changed over the years to take advantage of technology and social trends."
Keyloggers, which look like USB flash drives, can invisibly record every key stroke made on the computer.
They can automatically record all emails, conversations, chat records, websites visited, passwords and documents typed.