Once you import a photo into Lightroom, there is a copy of the photo on your hard disk somewhere. So deleting the photo on the camera card makes no difference, Lightroom still has a copy of the photo on your hard disk to work with.
You did not deleted the images previously imported into Lightroom.
Lightroom will only show images that exist on your card(shown in Finder/Explorer)
Could you also provide a screenshot of Lightroom Import window to illustrate the issue?
I don't know much about Canon DSLRs, but I know that one has two card slots. Is it possible that you are writing the same image to both cards, but only formatting one of them?
You are correct. The images were on the other card.
Many thanks for your assistance.