Unless otherwise you set the MAX Server Memory of SQL Server, it would try to utilize as much memory available in the server and hence utilizing 2 GB is not a big deal as SQL server loves more memory for optimizing things even though it doesn't have much activity as mentioned..
>> Is there any way to find out what SQL server is doing ? I'm of a mind to just stop the services for it as we don't use any of the reporting/monitoring functionality but I'd prefer to find the root cause if at all possible
Just execute this script in query window:
select * from sys.sysprocesses
which would list the activities happening in the server.
And regarding disk activity, if you have heavily fragmented databases, then this should happen.
Rebuild all indexes in your database to fix this one..