When you open a workbook and you change something in the workbook and you close the workbook, it will ask you to save the workbook. But sometimes, even if you have not changed anything, still the workbook asks to be saved even if you haven't changed.
Point 1 - Microsoft has provided the answer to this question at following link -
In case, this article gets removed or not being accessible, I have created a pdf copy of the above link which you can download from Save changes in _filename__ prompt even if no changes are made
Point 2 - The pointers given in the above article should be checked in your Personal.xlsb file also. You can rename Personal.xlsb to something else and determine whether Personal.xlsb is a potential source for this problem. Once determined the cause, you can rename the renamed file back to personal.xlsb.
To locate your personal.xlsb -
Please look into C:\Users\User_Name\AppData\Roaming\Microsoft\Excel\XLSTART (replace user name with your user name, you can directly reach here by copy and pasting following without quotes in Explorer address bar - "%appdata%\Microsoft\Excel\XLSTART\").
Now open the file and see if the problem disappears or not.
Note - Another alternative way to find XLSTART folder is
1. Open Excel
3. If Immediate Window is not visible, type CTRL+G. Same can be also be accessed through View > Immediate Window
4. Type "? application.StartupPath" without quotes and press enter
5. You will have your path below.
Point 3 - Another way to pinpoint the problem if above does not work is by process of elimination. Sometimes, many VBA controls / shapes do cause these problems. Move few portions of Excel sheet to freshly created sheet. Now, save and close this new Excel sheet and reopen and close and see whether this asks for Save As or not. Keep moving portions to new Excel sheet from old Excel sheet till you are able to determine the culprit portion.