As a TV news photographer myself, if they had me sign a release, especially to photograph a publically owned vehicle, for a legitimate news story, I know myself, my reporter, my ND would say thanks but no thanks, and walk away. I've never heard such a thing. Besides, it's a fully marked vehicle, that's going to be used for responses in public, and you're only shooting the non-descript outside. Id understand if you're photographing undercover FBI units, but this? What crawled up thier putukus and died?
Once the thing's in public, and in plain public view, it's open season to shoot the heck out of it.
Once the thing's in public, and in plain public view, it's open season to shoot the heck out of it.