If you close an image that's been changed since it was last saved as xcf or xcf.gz, you get a warning box,

    Save the changes to image 'img_7047-bw-columns.xcf' before closing?

    If you don't save the image, changes from the last 9 minutes will be lost

There are the following cases:

(1) File never saved as xcf, but has been exported
    In this case, if the exported file is up to date, jut warn that
    no xcf-format image was saved.
    (for extra points don't do this is the format is lossless and there
    is only one layer)

(2) File has not been saved or exported - warn as now

(3) File has been saved, and exported

3a saved is up to date, exported is not
    warn that the image has been changed since it was last exported,

3b saved is up to date, exported is up to date
   no dialogue in this case :-)

3c saved not up to date, exported is up to date
    warn that the xcf file is not saved, but mention that the
    image has been exported, and give both filenames.

One way for 3 might be a summary:

The image has been changed since it was last saved or exported:
file: foo.jpg
status: changed

file: foo.xcf
status: saved

The goal here is to avoid a dialogue that tells lies - e.g. it says changes will be lost but they are in fact in my final png file - and to avoid data loss - confusion between which of the 2 files is up to date.