If you are going to carry one, it should be visible. Along with other things in any vehicle, in an accident, it's a projectile, and really should mounted, that's why they usually come with mounts.
I was wondering where do you guys mount your fire extinguisher is your vehicl? I currently keep mine in the truck because id had to drill holes in the floor pan on my 71 rr to install the mounting bracket. Im thinking the trunk isn’t really a good place to store it because the gas tank is right under the trunk and if the gas tank caught on fire it wouldnt be impossible to get the extinguisher.