I`m assuming you are talking about this:
Important: For the noindex directive to be effective, the page must not be blocked by a robots.txt file. If the page is blocked by a robots.txt file, the crawler will never see the noindex directive, and the page can still appear in search results, for example if other pages link to it.
So Google tells you that if you add a noindex rule to robots, the crawler won`t access the page because of the rule. Google also says in other posts that crawlers will ignore noindex and nofollow rules regardless of where they are placed. That means that the crawler will understand the rule but still crawl the page, marking it as noindex. So yes, you will get this error from time to time even if you don`t have a header, but only a rule in the robots file. It happened to me, that's why I know..
And since when are we starting to trust what google says?
And I only brought robots.txt up because op said they checked everywhere on the page and didn't find a noindex tag, so I assumed they did check the source code of the page and I was just thinking out of the box..
