Why am I prompted to save changes after launching a previously inserted text file.

As the title suggests, I have a Word document that contains an inserted text file - by adding from file and displaying as icon.

I then open my Word document and launch the embedded text file by double clicking.

I then close the text file and close the Word document. At that point I am prompted to save my changes in Word. Although I have made no changes. Why?

Interestingly if I follow the same process but embed another Word document instead of a text document I am not prompted to save.