Red Flag laws, aka Extreme Risk Protection Orders (ERPO) are state laws that provide for a process of confiscating firearms that are accessible to persons...
We seem to be in agreement that it is a process, what seems to be the contention is if "pre-crime" confiscation can ever constitute due process.
But even that characterization can be challenged if given enough effort. Police confiscate items all the time before charging someone with a crime during the course of other investigations including computers, cell phones, vehicles, whatever might be pertinent. If no charges are filed (and no civil forfeiture invoked), the items are generally returned after a period of time. Ostensibly, someone has raised a concern about someone to law enforcement under these red flag laws, who must then undertake an investigation to determine if a crime has been committed or can be prevented. This is the closest parallel in my mind, since in both instances law enforcement must be able to make a case before a judge before taking any action.
As far as how long a gun owner can expect to be inconvenienced, from Wikipedia
How long the guns are taken away under these “extreme risk protection orders” depends on the circumstances, and can usually be extended only after another court hearing.
In some of the states that have passed these laws, not just anyone can raise a red flag. In Oregon, only people actually living with a person may file a petition, and in Indiana it is restricted to only law enforcement (Source)
I can understand someone's frustrations if they have their personal property confiscated, but the Supreme Court has upheld no-notice confiscation in other areas of law, and I don't believe just because things are guns that they would be treated any differently. For instance, in Calero-Toledo v. Pearson Yacht Leasing Co. (1974) the Supreme Court held (summarized by Justia):
[...] postponement of notice and hearing until after seizure did not deny due process, since (1) seizure under the statutes serves significant governmental purposes by permitting Puerto Rico to assert in rem jurisdiction over the property in forfeiture proceedings, thereby fostering the public interest in preventing continued illicit use of the property and in enforcing criminal sanctions; (2) pre-seizure notice and hearing might frustrate the interests served by the statutes, the property seized often being of the sort, as here, that could be removed from the jurisdiction, destroyed, or concealed, if advance notice were given; and (3), unlike the situation in Fuentes v. Shevin, supra, seizure is not initiated by self-interested private parties, but by government officials.
From the Federal Rules of Criminal Procedure (Rule 41):
(b) Venue for a Warrant Application. At the request of a federal law enforcement officer or an attorney for the government:
(1) a magistrate judge with authority in the district—or if none is reasonably available, a judge of a state court of record in the district—has authority to issue a warrant to search for and seize a person or property located within the district;
Possible crimes I can think of off of the top of my head that law enforcement can be investigating after the initiation of a red flag law petition: Improper storage of a firearm, Making terroristic threats, Domestic violence, Posession of a controlled substance, the list can go on. What law enforcement actually investigates depends on the situation and the content of the actual complaint. But there doesn't seem to be any difference, to me, between the initiation of this type of seizure and that of those done under so-called No-knock warrants. To the best of my knowledge the Supreme Court has not ruled directly on such raids, but the Department of Justice has published opinions asserting that right, such as this one from 2002:
After giving full consideration to these submissions, and having reviewed the
pertinent statutes and case law, we conclude that federal district court judges and
magistrates may lawfully and constitutionally issue no-knock warrants—i.e.,
warrants authorizing officers to enter certain premises to execute a warrant without
first knocking or otherwise announcing their presence where circumstances (such
as a known risk of serious harm to the officers or the likelihood that evidence of
crime will be destroyed) justify such an entry.
Summarizing the whole thing, judges already seem to have the authority to issue warrants that allow police to enter someone's home and take their property while conducting an investigation. All Red Flag laws seem to do is ensure that possible crimes concerning guns are treated the same way as other possible crimes that many times also include guns and increases the risk to police while enforcing the law. In some cases, it may even be argued that Red Flag laws may help protect gun owners by restricting who can actually file a petition.