Something is eating my indoor thyme plant at night and leaving little tiny pillets on the counter. What kind of insect does that indoors? No sign of anything during the day.
Drought Smart Plants reply: Well, first of all, this isn’t thyme, it looks like culinary sage, or Salvia officianalis.
The damage you see is typical of some kind of caterpillar, which based on the size of the holes is really small.
Your description of the frass is typical of a caterpillar.
I would put the plant outside, and spray with just plain water to dislodge the little character. Avoid using any kind of chemicals, as you most likely will be using this to flavor your food, right?
If you look very carefully at the underside of the foliage, you might be able to see a tiny green caterpillar – they are very well camouflaged in some cases. Simply pick off the leaf and put it outside so the caterpillar can still hatch into a moth or butterfly and continue its lifecycle.
Best of luck with your nocturnal marauder!