Hello, indeed it does appear that the volume efficiency show command only shows those volumes that have been "touched" by the volume efficicency command, either specifically enabled or disabled. If volume efficiency has been previously enbabled and then subsequently disabled, these volumes will then be displayed in the show output...
cluster1::volume efficiency> show -fields state
vserver volume state
------- -------- --------
vs1 vol1_dis Disabled
vs1 vol1_en Enabled
2 entries were displayed.
To confirm, I could only achieve this after I previously enabled efficiency on volume vol1_dis.