Leaf miner.

That was my firs thought. I have had them before. No lines, leaves looks like this in 24 hours. Nothing then this? Seems to quick, but it does look like beet leaf miner pics I found.

It's known as 'blotch' leaf miner, and you can see why.

FWIW, I remove the affected area of the leaf and smoosh really hard to crush the tiny larvae, then discard.

I have been removing infected areas and it works pretty good and leaves are fine.

Will sevin kill them when they are inside the leaf?

Pretty sure there is no pesticide that will reach the larvae inside the leaf. Squishing works well to kill them, IMPE.