as you mentioned,  symmetrix devices are being used in other storage groups. For me it's a good thing that you see this activity so you don't turn around and try to reclaim those devices, thinking they are no longer in use.
Hello  I decommissioned a host several days ago and removed it's zoning from the fabric however I noticed that the IOPS performance charts for the host's Masking View shows there is still some IO (see image) being generated for that MV. There are several other MVs on that same Storage Group, some of those hosts are still active, but I figured there shouldn't be any activity reported on the MV for the powered off host.  Can somebody explain where could this IO be coming from? THANKS!!  [IMAGE]
Masking View showing IO on powered off host