Why does the device's FingerPrint scanner scan erratically when no one accessed the device?

One possible reason to this would be the surface is dirty and require cleaning.Check and see if there is any residue left on the finger print lens. Sometimes a dirty finger print may result in such behavior.

The other possible reason was due to the environmental lighting's interference at the device's FingerPrint scanner which it will make the device auto-trigger to detect the finger.See the below as an example.

Proposed solution:

A) For the device's Firmware version 1412 or newer version,please change  'FingerPrint Detection Threshold' value from 300 to 1,000 under device's [Terminal Setup] webpage settings.

This value will allow the client to setup different sensitivity of the device's FingerPrint scanner under different environments.Once changed,click 'Submit' button to make the changes affect.See the below screenshot file as an example.

B)Also,please change 'FingerPrint Detection Method' from Fast to 'Normal'.Once done,click 'Submit' button to make the changes affect.

C) If the device's Firmware version is older than 1412,please set the 'Security Level (for Automatch)' to 'High'  under  the device's [Terminal Setup] webpage settings. Once done,click 'Submit' button to make the changes affect.