Someone reported cheating during a quiz, but I didn't catch it myself
You're the teaching assistant.  Inform the professor and let them handle the matter or offer you guidance.  That's their job.  In this case the evidence for cheating is tenuous at best, so I suspect nothing will be done against any of the students.  I've known more compelling cases of cheating to go reasonably unpunished, and one case of very blatant (and easily proved) cheating that was handled with just a stern admonishment and warning from the professor.  But you (and the professor) would rather the professor hear of the matter through you, than the possibly salacious gossip of the students.  

So what what really needs to be addressed is:

Always keep your eye on the students during exams/quizzes. 

As much as you want to believe that they're all perfectly moral and self-policing people (and quite probably most of them are), or that they can't possibly fit in cheating during "brief" windows, there will always be exceptions, and you owe it to the students who are such from giving such a clear opening to those who aren't (for whatever reasons).  You can't prevent all cheating all the time&mdash;it comes in a lot of forms, and some people are really good at it&mdash;, but this was easily preventable.

You may find yourself needing to apologize to the professor for this lapse, though I wouldn't get too worked up over that.  Most of us have made such a mistake at some point (although not all of us may be aware of it), and the minor value of the quiz helps take the edge off, such that a simple apology and recognition of the error is sufficient to make things right.