To the question of how I define "fail to secure".
I would define this as being "any firearm that was not under the immediate control of the owner that would permit unauthorized use".
Certainly the ideal situation is to put the firearm in some type of lockable container that denies unauthorized access as well as theft. Yes, I know that given enough time and resources any container can eventually be defeated, but that is not the point. The point is reasonable care to keep it from being stolen or used without authorization. Keeping it from being stolen is admittedly harder to prevent than unauthorized use w/o having a safe, but most any lockable box.....even a suitcase is better than the dresser drawer. However, I would rather have it stolen with a trigger lock than in a ready to use mode.
I recognize that not all can afford a giant safe. However, if you can afford a gun you can afford a trigger lock. Almost all new guns come with a free trigger lock, but use is legally not mandatory. Some will argue that the trigger lock makes the gun not readily accessible. This argument misses the point of "not under the immediate control of the owner". If you think you need your gun under your pillow at night, go for it. However, when you leave in the morning take it with you, put it in a safe/lock box or put a trigger lock on it.
Just as there are penalties such as "reckless endangerment" for improper discharge of a firearm and other activities that endanger the public, the same can be extended to incidents where firearms are left unsecured. Not real hard to craft some legislative language here.
I think unless the shooting public voluntarily recognizes that owning a gun also requires a means of preventing theft, law makers will make the decisions for us. I personally feel that a safe is mandatory and have bought a few less guns in order to buy the safe. I really can't think of a situation why one would not use a trigger lock it the firearm was not in their immediate possession and did not have a lockable container.