Will cannes food tops melt if dropped in boiling soup?
No, it won't melt to the point where it becomes liquid and mixes evenly with the soup. You cannot have consumed a melted top without knowing it. 

There are very few "everyday" metals which melt at below 500 C. Even tin (which I doubt is actually used for food packaging any longer) needs 231 C, while soup is at 100 C max (boiling point of water). If your top was metal, it wouldn't even have softened. 

Plastics have a lower melting point, but they are amorphous materials with a very wide transition range. This means, even though they can get quite soft at 100 Celsius, they will remain in one piece. A plastic top could not have melted and mixed with the liquid. 

Also, neither metal nor plastic is soup soluble. If you had had some material which melted into the soup (and that's purely hypothetical, after all nobody makes food cans out of gallium), it would have coalesced into visible hard lumps after the soup cooled down. 

If there is indeed a can top in the soup, you'll be able to find it, practically in its original state. And the food is still edible.