Growing beetroot, seeds germinated. After 10, 15 days of leaf showing, overnight plants disappear. Not slugs.

Question

I have planted packet of beetroot in two rows. One in early April. Seeds germinated, two leaves clearly showing 10 days later, overnight all dis-appeared. Re-planted, geminate, plants begin to show although a bit patchy. Put it down to dry May though kept watering. After a few days, plants have disappeared. Slug bait caught one or two small slugs at row end. Soil not disturbed at all. Turned soil over, no sign of chafer bugs or leather jackets. Forty years of gardening and never seen this before. Kitchen garden adjacent to back door. Not a case of plants wilting. They vanish altogether.

Answer 1

If you've been able to check your plants at night and you are certain there are no slugs or snails attacking your beets, you may have a hungry-rodent problem.

Hopefully this articles will help:
https://www.gardeningknowhow.com/plant-problems/pests/animals/vole-control.htm
https://www.gardeningknowhow.com/plant-problems/pests/animals/eliminate-rats-in-gardens.htm

Deer also enjoy newly sprouted plants.