Regarding Acid Arrow and MM:
You can cast
acid arrow at anything: a tree, an amulet, or the ring on someone's finger. You can cast it on a
spiritual weapon, a figment illusion, or just straight up into the sky. This is because it has the "Effect: one arrow of acid" line in the description.
Magic missile is different. It can only be cast so that it targets a creature. It's like
charm person in this regard. You can't just cast
charm person up in the air, on an amulet, or even on a Giant. If you try to do these things the spell just won't do anything. Same with
magic missile. Cast the
magic missile on the air, an amulet, or anything else that it cannot target, and it won't work.
And, magic missile cannot seek the real caster. It says in the description of the spell itself that you have to pick out a particular target. You can't say "Hit the wizard" and the spell will do it. You must point out which figment(s) the spell will be hitting.
Dannyalcatraz said:
I was assuming just that one "kobold," so "that kobold there" is a specific target.
The spell doesn't care what you call the target. The spell only cares if you pick out a particular creature and if that creature is a valid target. I can see a guy with an apron and cleaver and say "I'm casting
magic missle at that butcher there," and just because he's not a butcher doesn't mean the spell suddenly doesn't work. If it did it would be a great way to detect all sorts of things. "I'm going to cast
magic missile on that evil guy there." Or, "I'm going to cast
magic missile on that serial killer there." There's no divination involved.