I have never really done a survey of airports to note the signage.
As far as I recall during quite a few years of extensive travel, it has been a consistent and very often repeated message from all levels of the travel industry, from travel agents, airline checkins, airport announcements etc. That is: - always lock your luggage and your belongings, - do not leave any of your belongings unattended, - watch out for thieves at the airport and safeguard all of your luggage - never accept and carry a parcel from anyone else unlese you are completely sure what is inside it
I would imagine that many airlines, over the history of the airline travel industry, would have found that most people who are functional enough to buy an airline ticket and navigate their way to the airport, probably are functional enough to understand that they should also put a lock on their luggage.
Of course they would be wrong in some cases, but overall I think their assumption would fall within the bell curve of intelligence.
