Sigh- I feel like people's legal research department.
A police officer who discovers a weapon in plain view may at least temporarily seize that weapon only if a reasonable officer would believe, based on specific and articulable facts, that the weapon poses an immediate threat to officer or public safety. United States v. Bishop (C.A.6, 2003), 338 F.3d 623, 628; and see United States v. Frederick, 2005 Fed. App. 0864N, No. 04-5614 (some objects are "dangerous in themselves," and the test is not whether the officers in fact feared for their safety, "but whether a reasonable officer informed of these facts could conclude that the weapon posed an immediate threat to officer or public safety").
Arizona v. Hicks (1987), 480 U.S. 321, 107 S.Ct. 1149. In Hicks, police entered a home to investigate a shooting, and, suspecting that expensive stereo equipment in the apartment was stolen, moved some of the stereo equipment to observe and check the serial numbers on the equipment. The U.S. Supreme Court held that recording the serial numbers of the stereo equipment was not a seizure because it did not interfere with a defendant's possessory interest, but, moving the equipment to see the serial numbers constituted a search.
Once an officer had a valid reason to secure and possess the handgun, he was able to obtain the serial numbers without violating appellee's Constitutional rights. See United States v. Lara (C.A.9, 1991), 932 F.2d 973, (deputy testified that when nonofficer personnel are present at a search site, weapons found in plain view are disarmed as a safety precaution, and therefore, having a valid reason to pick up the weapon and lawfully possess it, the decision that officer was entitled to examine both guns for missing serial numbers was not erroneous); State v. Wilson (1998), 93 Wash App. 1025 (there is warrantless seizure exception for officer safety, and where officer is lawfully in possession of gun, officer is lawfully in possession of serial numbers; plain view analysis of Arizona v. Hicks is inapplicable when Hicks involved extent to which officers can expose to view items in their presence, but not in their possession); United States v. Watts (C.A.8, 1993), 7 F.3d 122.
So, to sum things up, once an officer is aware of a gun, he needs an articulable reason to believe that it is an IMMEDIATE threat, not merely a possible one, to temporarily seize it, and that reason will be determined by the court. The question here is whether or not the weapon presented an immediate threat to the public or to the officer. IMO, in this case it did not.
One thing is for sure, if the cop doesn't know it is there, he can't take it. Once he does know, he might be able to seize it. If your state law does not require notification, don't notify. ANYTHING you say to a cop can be used against you. Keep your mouth shut. That is not anti-cop, just prudent advice.