re: How fast do you lose muscle vs. fat when in a strict calorie deficit?

This is anecdotal, I'm not quoting or referring to any science backed data here. I'd have to look it up what the research says on how many hours, but of course it matters what a person's diet and activity was like baseline before falling away from regular routines for a few days.

If a person was being active as you describe, (not just lying down in bed, perhaps sick for 2 days), and they are taking in some calories, but not enough calories to reach baseline and not their routine macro ratios either, I don't think they would experience any muscle catabolism. And if this person has some body fat, that will probably get hit first after carb stores are depleted. Now I'm assuming that this person is sleeping "normally" for them, and whatever activity they're temporarily engaging in is not too taxing, it's just "doing stuff".