Welcome to the Q&A Forum

Try putting the URL into Google and see if you find any pages linking to it. I knew a company that created a test site that was a copy of a live site (made with a specific hosted CMS). Didn't exclude the test site in robots because "we all know we won't link to it so it'll be ok". Site got indexed, and it was because a person at the company was having problems with the implementation of the test site, went to the help forum (which person didn't think would be indexed) and posted the URL to the test site. I found the above by just putting in the URL of the test site into Google, and I saw the post in the help desk. You might try the same to see if somehow there is a rogue link.