It appears that the padlock in question is generated as an emblem by gnome-disk-utility and it's
the gdu_encrypted_lock.svg and gdu_encrypted_unlock.svg that I can't tell the difference between at that size.
The problem stems from the fact that:
1) When used as an emblem the padlock is pretty small (the shackle is probably no more than 4 pixels high for me).
2) The shackle is a white/grey and as an emblem it is sitting on what is typically a white/grey disk.
I spent some time tracking this down a bit more.
It appears that the padlock in question is generated as an emblem by gnome-disk-utility and it's lock.svg and gdu_encrypted_ unlock. svg that I can't tell the difference between at that size.
the gdu_encrypted_
The problem stems from the fact that:
1) When used as an emblem the padlock is pretty small (the shackle is probably no more than 4 pixels high for me).
2) The shackle is a white/grey and as an emblem it is sitting on what is typically a white/grey disk.