CAD tip # 8408:

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:

(acad-push-dbmod)
_Zoom _E
(acad-pop-dbmod)
_Close