Thanks all for the responses & suggestions.

The good news is, no further damage after another night. For want of a
better idea, I had doused the seedlings with appropriately diluted
Safer's soap. Don't know if that had any effect. Was also going to
sprinkle with rotenone but chickened out.

(Apologies to politically sensitive chickens.)

The soil mixture varies from pot to pot - including some garden soil,
some sphagnum, commercial potting soil, vermiculite etc, etc. Not
anywhere near uniform enough for a cotrolled experiment! Isolating
each pot, or the whole lot, under a sealed cover would have been
difficult so I didn't try - but the idea was sound; thanks.

As an added variable, I've been moving everything outdoors under cover
on the warmer days.

And no, there is no cat with access to the seedlings. Their indoor
environment is in my garage/workshop. But I've been blaming the
neighborhood felines for chomping down the greens on my outdoor
garlic. Any comments?

No trace of slug slime on either seedlings or garlic.

Aren't gardens fun !!!