In my humble opinion that is not a bug at all. I do not want to prove, disprove or start a lenghty debate. It is just unlikely.
I can extend that beyond Marketo as one of my roles as volunteer for a non-profit is maitaining its website. I often find that in Drupal's logs. (
Drupal is the leading open source content management system).
The anonymus visits, if not pure spambots, may be web crawlers refreshing the indexed content. That also generates a well defined pattern.
I am available to review the details through a Support Case. That woudl ensure sensitive data is not exposed to the entiire Community.