I agree that the police would probably just arrest and sort it out afterwards. I'm slightly more confident about the courts potentially looking into the legality of a detainment or arrest.
Maybe we can figure out the correct legal response by comparing other cases to this hypothetical one.
In
Florida v. J.L. an anonymous caller reported a minor carrying a concealed handgun. The caller accurately described what the minor was wearing. The cops show up, do a Terry stop, find the gun, and arrest him. The Supreme Court says the police lacked RAS to even detain him because they declined
...to adopt the argument that the standard Terry analysis should be modified to license a "firearm exception," under which a tip alleging an illegal gun would justify a stop and frisk even if the accusation would fail standard pre-search reliability testing.
We can agree that if I'm standing outside a library with a concealed weapon and somebody anonymously reports that I'm carrying a concealed weapon without a license that the police would be unable to detain me. If I move inside the library does this change at all? I don't think so, the police still can't detain me because they lack RAS. There's a Second Circuit opinion called
U.S. v. Ubiles that echos this logic (and probably lots of others that I am unaware of).
So now we're concealed in the library. What happens when we start open carrying? Well, for one thing, now the police are absolutely sure that a firearm is present whereas before they only has an anonymous tip. Do the police have RAS now? In
Delaware v. Prouse the Supreme Court said that a driver of a motor vehicle cannot be stopped solely to check their license:
Except where there is at least articulable and reasonable suspicion that a motorist is unlicensed or that an automobile is not registered, or that either the vehicle or an occupant is otherwise subject to seizure for violation of law, stopping an automobile and detaining the driver in order to check his driver's license and the registration of the automobile are unreasonable under the Fourth Amendment.
It would seem that the same logic could apply in this case. The police could stop you if they had RAS for something else, but the mere presence of a firearm doesn't grant RAS.
Where my logic starts to get iffy is the fact that a library is a public building and perhaps the courts would find that it is a "sensitive place" (from Heller), and thus the ability to carry a weapon there is a privilege and not a right. Perhaps the courts would be more willing to grant a "firearm exception" in that case, but perhaps the courts would think that since licensed individuals can carry firearms in a library, that it isn't truly a sensitive place, and thus is the same as any other place a firearm may be carried.
I think the weak point in my argument is the sensitive place bit. Thanks for the feedback.