I have signs on each side of my van.."Beware of Dog".  I have 3, but one who loves to come along everywhere and he really thinks it is HIS van..Chesapeake retriever, about 90 -100 lbs.  He gets kinda "Agro" whenever anyone comes near, so I try to park away from foot traffic if I have to stop at the supermarket or something and he is along...
  On the other hand, I often park in almost deserted places, then take off on my bicycle for a few hours of riding without traffic, leaving the van without any worries whatsoever..I don't think anyone would be dumb enough to read the signs on the windows, then stick their hand inside, with a hundred lb. yellow-eyed snarling monster doggie inside..I could probably park in an alley in downtown Tijuana or NYC without worries...
  I don't even have to bother with the locks..
  Don Hanson

  However, he did scarf down a whole family-size "Take-N-Bake" pizza while I went into the bank for about a minute..Guess a cat wouldn't do that...