Any modification of the drawing database, including changing system variables, results in the save prompt when you close the drawing. Such changes can be even triggered by your automatically loaded LISP routines - in this case exiting any drawing would invoke the dialog "Do you want to save?".
This AutoCAD behaviour is controlled by the DBMOD variable. This variable is reset by SAVE and set by any change in the drawing. Please note that even a zoom or pan changes system variables stored in your DWG - and so sets the DBMOD. If DBMOD>0 on close, the save prompt is displayed.
There is a method to reset the DBMOD value without saving so you can avoid the Save prompt.
You can save (push) the DBMOD status - especially the zero status - with the (acad-push-dbmod) function. Then you can perform any operation (zoom, edit, create...). After it the function (acad-pop-dbmod) resets the DBMOD value to the original (pushed) value - e.g. 0. Close will NOT invoke the save prompt in this case.
E.g. a script:
will complete without prompts.
You can also use this method if you are changing some system variables in your ACADDOC.LSP and you don't want to make all your drawings automatically "dirty" - subject to the save prompts.