For my case , I got warning about suspicious activity on one of the API key (NOT the same key that run the CodeBuild either)
The issue was raised (via email) about 8am. Then 11am. i doing the CodeBuild and notice the "0 build in queue" error. The issues on the suspicious API has been raised about 10 days earlier, and i only "inactive" them. So i guess what i got today is second warning.
So this time i "delete" the exposed API key. And just about the write to AWS support. I got email Thank you for removed the exposed API key. And CodeBuild just work fine after that.