Well if I ever see one it will mean the lock is unmanned and I need to work it myself. I thought the current system of guess if I'm here or not prevailed.
Perhaps you should have asked the same question without the orange circle which means the lock is manned - they might be rarer!
A bloke I knew years ago had a little 18ft fishing boat with a Japanese flag on the back. Finally, after several weeks I couldn't stand it anymore and asked him why they had that flag on their boat. He explained that if a girl asked him about his flag he was going to say "Just Nip on".
Sorry....but its true.