'warrant alarm' means there has to be some specific articulable action....
'causes alarm' is a much lower threshold and someone may be alarmed by the mere sight of a firearm, but the actor did nothing....
This may have been mentioned already, but
this article explains the two terms well.
Examples based on the law here in Colorado:
Open carry may cause alarm in citizens unused to seeing it. Cause alarm is not sufficient grounds for LEO intervention, primarily because OC is legal here in Colorado.
Brandishing constitutes "warrant alarm," and meets probable cause requirements for LEO intervention, although allowances are made for the circumstances (defense of self/others in the face of immediate danger).
I do NOT agree that the type of weapon carries warrants alarm, "reasonable person arguement" or not, as that's a slippery slope (fallacious) argument, at best.
I do agree with the author's last two paragraphs:
Conclusion: We neither advocate nor discourage open carry. The advocates maintain that exercising such a right educates the public including law enforcement. Since most people fail to appreciate the level of safety provided by those of us that carry concealed (out of sight, out of mind), carrying openly is a visible reminder that citizens help to deter crime. Open Carry advocates have been writing to local enforcement and many Departments have published training memos in order to avoid stops that violate the rights of armed citizens. See OpenCarry.Org.
On the other hand, there is something to be said for blending in and being prepared to react to an emergency without prospective assailants identifying that you are an armed citizen. All of us should be getting trained in weapons retention and practicing our gun handling skills. Meanwhile, it is good to know that we can get in and out of a car without worrying about what will happen if someone calls 911 when a weapon is inadvertently exposed to the gaze of an alarmed bystander!
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