So by your description of no complaints, this example has nothing to do with red flag laws.
As far as I know the red flag laws may entail not just complaints or reports, but anything that raises concerns for the authorities. It's possible that the group or political movement someone is associated may count as being a concern. If someone is associated with a radical organization, citizen militia, etc, that may be cause to raise a red flag (it probably should.)
However, the issue is seizing someones property without sufficient evidence. A complaint alone is not sufficient evidence to seize someones firearms....malicious intent by the complainant, etc.
I don't think anyone would complain about action taken on good reason based on evidence.