"Someone" is any user that downloads a file from a shared link while not logged into Box.  You will receive these updates if you or one of your collaborators has enabled “Open Access” shared links to content you own or have access to.

 

"Someone" is also shown if the user is collaborating in a folder that has the "Hide Collaborators" option turned on

 

For added security reporting, Admins of Business and Enterprise accounts can track the IP addresses associated with each notification referencing "Someone" within the Admin Console.