I came across this today and it took me quite a while to figure out what happened. I migrated a vSphere 5 environment from vNetwork Standard Switch to vNetwork Distributed Switch. Afterwards I deleted all old Standard Switches, but I had two Portgroups in Home->Inventory->Networking with Virtual Machines connected to them. Strange, as the vSwitches were gone.
After checking logs and playing around I found that these VMs had a snapshot. After deleting the snapshots the orphaned Port Groups disappeared, so I can now head off to HP Discover in Vienna.