When customer opened an existing record and then tries to move away from the firm, without making any change, still the browser gives a message popup that 'Do you want to save changes' and askes for Save or leave.
This may have caused by a UI Policy action, which has a checkbox for 'Clear The field value' checked.
While on load, a UI policy was hiding a few fields, but at the same time clearing the value. This is triggering an internal this.modified call as the value of these fields are getting changed on the form. That is why when the user is trying to navigate away from the form, it is asking the confirmation popup, as in backend for these fields the values are changed.
The customer needs to review the UI Policy actions. The easiest option is to uncheck the 'Clear The field value' field.
You can find the source of this by searching for this.modified in the browser's Developer tools search. Once you find the scripts that are making this.modified=true, you can put breakpoints, run the debugger and lookup through the stack trace to get the Client script or UI policy name which is responsible for making the modification. Some helpful resources mentioned below:
Also, refer the below product documentation on UI Policy: