storage.googleapis.com - What is this web filtering error for?

I have a Fortigate 60E and last night it came up with a number of web filtering alerts through the night from a few PCs and 2 servers but not sure what it is other than something Google related. Here's part of the error below and the site blocked is storage.googleapis.com but not sure why the PCs are accessing this randomly at night all of a sudden?

service="HTTPS" hostname="storage.googleapis.com" profile="default" action="blocked" reqtype="direct"

We do have Google Drive on the PCs not the servers though so bit lost, any ideas?