I am a cop of many years. seizing property without a warrant requires it to be legally contraband, or evidence of a crime. If a officer was to try and do that, let him, then go to a Judge's office at once, or even police headquarters and ask for a Supervisor to meet you there. Ask for your property to be returned at once, as no charges were filed, or to show the item contraband.
I had it happen once as a kid. I got my knife back pronto. Most cops are Pro Second Amendment!