Do you remember this thread? In it I showed photos of parasitized hornworm with grey spots where the wasp larvae are under the skin, coming out. So it's possible those scattered spots are signs of emerging wasp larvae.
Subject: Thought you could eat my tomatoes and get away with it, eh?
But I agree with you, Lindsay, that hornworms have their distinct diagonal marking even when in the younger instars. I think I have seen the caterpillar in your photo before... maybe in Bringing Nature Home by D. Tolmy. Do you have that book? I can look later.