We have this test.php sitting around in a forgotten folder. It is a script which just sends an email to our site admin.
We never had a page linking to it. It is not indexed by Google. It does not exist in the Internet Archive Wayback Machine.
But every now and then it gets crawled by ia_archiver.
I wonder how it got indexed. Could it be because of the Alexa toolbar installed on our computer? Does Alexa index our personal browsing history?