With all the talk about red flag laws in the interest of preventing mass shootings, it is instructive to look back on what the Supreme Court has said about warrantless searches in their recent history. The Court ruled unanimously that the search and seizure were unconstitutional. Something about due process, warrants, and the proper role of police while conducting something called "community caretaking".
In 2020 the Court ruled on a case our of Rhode Island. It seems that a man and woman, married, were having a verbal dispute in the home. The man, at some point, produced a firearm, put it on the table before him and said "Go ahead and shoot me. Get it over with." or words to that effect. The woman left the home to spend the night in a local motel. The next morning, the woman could not reach her husband by telephone and called the local police to perform a welfare check. She told the police that the husband might be suicidal.
The police responded, and in the course of the response, seized two handguns from the residence.
The case is Canglia v Strom and you can red it here. While the ruling or opinions never say "red flag law", it talks at great length about how the police react to functions that are not strictly law enforcement related.
Held: Neither the holding nor logic of Cady justifies such warrantless searches and seizures in the home. Cady held that a warrantless search of an impounded vehicle for an unsecured firearm did not violate the Fourth Amendment. In reaching this conclusion, the Court noted that the officers who patrol the “public highways” are often called to discharge noncriminal “community caretaking functions,” such as responding to disabled vehicles or investigating accidents. 413 U. S., at 441. But searches of vehicles and homes are constitutionally different, as the Cady opinion repeatedly stressed. Id., at 439, 440– 442. The very core of the Fourth Amendment’s guarantee is the right of a person to retreat into his or her home and “there be free from unreasonable governmental intrusion.” Florida v. Jardines, 569 U. S. 1, 6. A recognition of the existence of “community caretaking” tasks, like rendering aid to motorists in disabled vehicles, is not an open-ended license to perform them anywhere. Pp. 3–4.
I am certainly no attorney, but a full reading of the holdings and opinions in the link above would lead to the conclusion that red flag laws might violate the Fourth Amendment.