I know that, because I just copied the catalog without negatives, however, I would like to get rid of the message. How can I do that?
You get the "missing file" message if
a) either the image has been deleted outside of Lr,
b) or the image has been moved outside of Lr (via Mac Finder / Win Explorer or via Bridge).
In case of (b) right-click the image / folder and select <find missing image/folder> then navigate to the correct location and click on the file/the folder.
If an image has been deleted outside of Lr you can't get rid of the message unless you remove the image from Lr. Right-click the image and select <Remove>.
Did you ever get a real answer to your question?
I have the same issue - I've exported a catalog to my laptop. I KNOW the original won't be found. I just want to get rid of that bloody message.