Well it must be more than a cup of coffee - I'm guessing the owner works nearby because I've waited until it finished once last week and they didn't come back while I charged. It was finished another time this week so I didn't bother replugging. Then this morning which was earlier than the last two days no one came back to see what had happened.
So there is general agreement that it is okay if the charging is finished to completely unplug the other car - agreed. I am a bit surprised at the rest of the comments, but I will consider them - to me if they clearly aren't there and have left it to run its course, they have no priority over my use - just like saving a chair at poolside. If there is another chair (L3), I'll use it, otherwise their towel gets set aside nicely and I'll use the chair - at least until they come back (That ought to stir up a few more comments.). To me, plugging them into L2 is not as good as reconnecting the L3.
Given the lack of response when I have unplugged the empty LEAF, I'm of the opinion they are in no hurry and I'm not inconveniencing them significantly. Thanks everyone.