Of course going hiking means taking a backpack; are you going to risk destroying your carnet in the rain, sweat, heat and general crushing that the contents of a backpack have to survive??
If the truck's gone the carnet may have gone with it...
You're in more danger of coming back and finding it empty. Polaroids are good, they accuse the person you left responsible, when you come back and find the truck untouched, you can give the pic to the guard, he'll be happy.
A hidden switch on some solenoid wires or other knobbling device would stop it wandering.