From the Wikipedia page, emphasis mine:

In the United States, a red flag law (named after the idiom red flag meaning “warning sign“; also known as a risk-based gun removal law,[1]) is a gun law that permits a state court to order the temporary seizure of firearms (and other items regarded as dangerous weapons, in some states) from a person who they believe may present a danger. A judge makes the determination to issue the order based on statements and actions made by the gun owner in question.[2] Refusal to comply with the order is punishable as a criminal offense.[3][4] After a set time, the guns are returned to the person from whom they were seized unless another court hearing extends the period of confiscation.[5][6][7]

Intuitively, it makes sense the police would not be able to search someone's home for guns without a judge's permission. It would be hard to say that there was a compelling emergency just from going through things that someone had said or things that had been said about them.

I didn't see a federal supreme court case that ruled on red flag laws specifically, but it sounded like there were some state supreme court rulings that found them unconstitutional. So it is at least contentious whether they meet the strict scrutiny standard or not.