While I agree with the notion, I don't necessarily agree with the course of action.

I feel it would be better to leave a leaflet or a note under their windshield wiper and then call the local authorities. I see Lady's Human's point and do agree to an extent.

I agree 100% about your notion, but maybe just not about the stickers. I think it's a good idea, just maybe not the best way to go about doing it. While they may be breaking the law, you never know that one person could pin you for vandalism, and unfortunately that would probably get more attention and you'd get in a lot of trouble. Just leaving a sticker and going about your business will annoy the person, but chances are they won't learn anything from the experience. Calling the cops is a much better way of handling it, I think, because even if they don't think it's a "big deal" to leave their pets in an unattended car in the heat, I'm sure most people don't want to get into any legal trouble and will probably learn some sort of lesson, if not think twice before doing it the next time. And also, there's no way that you can be legally in trouble for anything (vandalism, etc).

I know here if I were to do something like that, even if the dogs were suffering, if the cops were called and both parties were present (myself and the owners of the dogs) I know that they would get a slap on the wrist and I would get a fine for vandalism. It's sad, but unfortunately that's just how it would happen here.

That having been said, yes, I have left my dogs in the cars with windows cracked while I ran inside a store or did whatever. I would never do it in a heat wave, even for a minuite, and I would be smart about it. But I don't think when I do it it would be considered "animal cruelty" or "neglect". So yes, I do it, have done it, and will continue to do so, but at the same time if I know I'm going to be making a couple of stops along the way and it's super hot out then I won't bring my dogs. Period.

However on kind of the same note, I have a word document saved on my comp for when we have visitors staying in our apartment complex that don't know how to park... Every time I come home to work to find there aren't any available parking spots because some IDIOTS don't know how to park properly, I run upstairs, print a few off, and slap them under their wipers...