Mice, voles, squirrels, rabbits, woodchucks, chipmunks, some birds, deer, cutworms, some slugs/snails, sometimes raccoons and opossums will all snack on sunflower seedlings.

Interestingly, sunflower and legume seedlings are among the most popular with rodents and birds. Lots of protein. Probably because the seeds are large, and what's in the small seedlings are mostly what was in the seed.

Gerbils love sunflowers

Keep in mind it was most likely at night. I walk by them 20 times a day they were there yesterday at sunset and gone at sunrise. No stem left (cutworms leave stem). No deer, no groundhogs. Birds all over constantly - no issues for 2 weeks. mice vole seem by far the most likely.

Coon, rabbit and possum a slim chance but location with other plants they walked by unlikely.

Slugs nibble on some and have - but never ever ate an entire one completely in 12 hours.

Sounds like you already know so why are you asking us? Still waiting for that asparagus seedling picture.

Trying to get more ideas. I didn't think of rabbit. Thats very possible.