Wow, sunflower seedlings? They do look somewhat like sunflower leaves, and that would perfectly explain why they are coming up near the sidewalk. I looked hard around them to see if I could see any seed pods but I wasn't able to find any. That's a deduction worthy of Sherlock Holmes! Of course, if the seeds were roasted, that should kill them,no? But people do eat raw (not roasted or salted) sunflower seeds, don't they? Isn't that what people put in granola? I wouldn't put it past a pedestrian to dump a cup of granola snack on my lawn.

yes it can. When in So Calif, my yard ended up with a mound underneath it, a mixture of sprouting seeds, hulls and strange growth. I used to purchase the least expensive bird seed I could find. Since I moved to Idaho I have been using a no sprout bird food without any issue. I now have 3 feeders (over grass and planters). It's more expensive, but I notice I have no waste product underneath.

If you planted lemon seeds the resulting plants are lemons. They will not be true to variety but they will always be lemons- nothing "non-lemon" about them. Don't try and judge a plant by the appearance of the sprout- it's way too soon to tell. Of course it is always possible that a weed seed got in there, especially if they are outdoors. Birds deposit all sorts of seeds everywhere.

They were probably "anting"-birds place ants in their feathers so toxins sprayed by the angry ants can kill their feather mites