If you open several files they can be marked as changed (indicated with an * in the title bar) even if you have made no change. To reproduce:
-
Start a new worksheet, type any letter, save it. You will see the filename in the title bar
-
open another worksheet, type anything
-
switch back to the first file
-
switch back to the second file. if you have arranged them side by side you will see an asterisk after the filename.