PHP Inspections v2.3.18 installed.
PhpStorm 2017.3.6
Build #PS-173.4674.46, built on March 15, 2018
JRE: 1.8.0_152-release-1024-b15 x86_64
JVM: OpenJDK 64-Bit Server VM by JetBrains s.r.o
Mac OS X 10.12.6
I ran an inspection of my composer.json. Below is the screenshot

Hi @craiglondon: doesn't look like something from us. Do you have composer.json plugin installed? Perhaps the inspection belongs to it (I guess).
I think the same. I have installed it soon and I see the same issue. So I think that it is related to the composer plugin.
Thank you. I do have this plugin installed as well PHP composer.json support v 1.0.25 and when I disabled it, those messages related to annotator went away.
@craiglondon : you are welcome. I think it should be also possible to disable inspection which causes the issue and keep the plugin enabled - it helps pretty much.
Most helpful comment
I think the same. I have installed it soon and I see the same issue. So I think that it is related to the composer plugin.