Lead can get into food if someone in the supply chain is trying to cheaply weigh down a product to make it feel heavier as if it has more food than it does. See lead in wine controversies.
You would have to put a lot of lead in food to make an appreciable difference in mass. Lead in wine, as I understand it, is about sweetness and preservation, not density.
I really do not think anyone is adding lead to food to increase the weight when you could use any metal compound with a roughly similar density like iron oxide (which even has an E-number) and thereby not obviously poison anyone for the sake a few grams and get yourself discovered immediately.
And if you wanted to weigh down the container, lead costs more then iron per kg: just make the existing iron-based can thicker.
