Hi,
Just some ideas:
you can certainly see machines hitting dropbox URLs from the custom reports. Make sure relevant columns are enabled. Just filter by distant URL and use "contains" "dropbox". You will see number of hits but unfortunately no measure in terms of the amount of traffic.
If possible you can also find out the IP addresses of the dropbox servers and create a filter based on that.
If SWG has been integrated with Active Directory and authentication is implemented, you can then see the login name that corresponds to the offending machine as well.
Regards,
Federico