I cam across this issue when running bosboot command. It appears that you can safely ignore it.
It is the reference to the old disk which was part of mirror rootvg and that has been removed. If you want to avoid this issue for the future, don't forget to run this command against the removed disks, prior to remove the device: chpv -c <diskname>. After a reboot this message will no longer appear.

# bosboot -ad /dev/hdisk3
unable to identify boot disk
unable to identify boot disk
unable to identify boot disk
unable to identify boot disk

bosboot: Boot image is 41368 512 byte blocks.

# bootlist -om normal    
hdisk3 blv=hd5

# bootinfo -v
unable to identify boot disk
hd5