More leaf nibbling

[Vanisle_BC]

I'm having seedling leaves nibbled by something small. The damage is pretty insignificant at first but builds up to major over a few days. I use Bt and don't see any sign of caterpillars. Don't see any flea beetles. I do have occasional slugs but I think they would destroy small plants at the first go. Any suggestions?

[rainbowgardener]

It does look like slug/ snail damage