I live in California, and I've noticed that everything has a CA Proposition 65 warning label. When I look into it it's generally lead that has bioaccumulated in the plant. What I can't find is how much or how widespread it is in the food.
Lead naturally occurs in the environment, depending where, in the world you are, from 2 to 20 even UpTo 50 ppm. Industrial/commercial sites are common to have up to 150ppm. Contaminated toxic sites way over that, over a thousand. We could go on. But the swabs work as a starting point.
I remember seeing an article a while ago, where some local councils here specifically recommended building raised garden beds for veggie gardens. They apparently found, that in cities/towns older than about a hundred years, the risk of soil contamination can be pretty high, the main suspects being lead and arsenic. Raised garden beds is an easy way of eliminating that risk.
I'd recommend raised beds to anyone who lives in a suburb or city that's even a few decades old. The particulates and oils washing off the street and down roof shingles alone introduces plenty of contamination. Very rainy regions like the Pacific Northwest have to build rain gardens [1] everywhere to filter out baseline levels of suburban pollution, otherwise their water gets really bad.
Also I've read that sunflowers are really good at pulling up heavy metals, and all of the bags that I purchase have warning labels on them. I'm curious if this is a real concern!
I assume that I could grind up the food, test the slurry and then maybe do a more in depth test involving putting the food in a kiln and testing the ash for lead (assuming you use temps low enough to not vaporize the lead, below 1750C).