seedlings nipped off at the soil, uneaten

I grew borage from seed and planted about half of them out when they were sort of palm-sized with two pairs of true leaves. Some in the polytunnel (my first season with indoor grow space) and some outside. Nearly all of the ones I planted, both indoors and out, have been nipped off right at the soil level but otherwise undamaged, just left lying there.
Ive got broad beans coming up in the tunnel and a few days after the borage massacre, about 1/3 of these were also nipped off at the ground, but left lying there. I've never really had this happen before.
There is other stuff around and growing, both inside and out, although not tons.

Some slugs have been spotted, not much damage yet, and this doesn't look like slugs to me.

All but one of the borage seedlings were NOT near any mulch or woodchip.

There certainly are mice in the area but not really infesting, as far as I can tell.

What on earth is going on?

Could it be cutworms?

It sounds like cutworms to me also...try digging around the cut stem a bit and usually the worm will still be there. I almost always find one in the soil nearby.

Yep, that seems likely, I have seen some of those pupae cases in the compost pile and in the lasagne bed. I had not noticed any in the tunnel, but I guess that's likely. So what do I do? Some sites said basically, just assume a certain % of loss, some suggested eggshells.coffee grounds/DE, and some said cultivate heavily - bit of a problem there since i am aiming for no-dig/little-dig in much of the garden... any suggestions?

you can make cutworm collars around the stems about an inch below ground and above. I use heavy paper, some use foil. Sometimes I use a 3-4 inch circle around each plant set into the ground an inch or so. That works easy for the plants you are setting out. The ones coming up from seed though are more difficult. At the first sign of damage i dig around for the worm and feel like that prevents a lot of loss. Read up on their life cycle for your area.. Here I think I remember that there were peak times when they were busiest, May, I think, and if you could avoid planting young tender plants then, you avoided a lot of damage. The 'greasy' cutworm that we had the most trouble with is the larvae from a moth that lays eggs in overwintered green stuff...dead nettle and chickweed and clovers were all suspect:)