I don't think there is any solution apart from not leaving dogs in a car. We try not to leave Domino in the car (though I'm not sure a burglar would dare go near the car when Domino's barking at them), if she is left it's only for a few minutes to pop in a shop and that would be in busy place. I always lock the car though as I would whenever I leave the car. I'm not sure putting a sign up saying they're neutered would deter a dog snatcher either. So sad that dogs are taken and then probably not treated very nicely.