Its something that can happen to anyone, you could even simply lose luggage with the laser inside and it could end up on a dump pile where someone innocent may pick it up.
I suppose it would be best to remove the batteries from powerful lasers whenever you carry them around and not using them. That way, if they fall out of your pocket or something, someone can't just pick it up, look down the barrel and press the button.
If you laser is powered by lithium ion cells (18650s or something) its not very likely some random person would have a suitable battery at hand to give it a go either.
One problem is that people are more stupid that you would think. Many people will power on a flashlight pointing it straight at their face to see if it works. This wasn't really that dangerous until the multiwatt led flashlights came a long, but with a laser its certainly a very bad idea.
I doubt warning stickers will deter many people either... everything comes them, up to the point stating a butchers knive is sharp and should not be used to clip your toenails.
One of my favorite quotes: Imagine how dumb the average person is. Now realize that half of them are dumber then that.