My car sticker says caution k9 because that's how it came, it was on clearance so over 50% off and I didn't feel like special ordering something else. The "K" is actually falling off. The campus security where I work likes that my vehicle is marked. Obviously when the dogs are inside, the windows are down an appropriate amount, but you can't actually see dogs or crates unless you're real close (especially though the back) because it's a van and they aren't stacked or on a platform. There are some exact copies of my van that park in the same little lot (I tried to get into someone else's once, same model year and same interior as mine). They do not mind me having dogs in our lot because they are quiet and they are taken care of, but they like to know which vehicle is mine without having to approach and look in (and I prefer that too, even though the dogs wouldn't react unless you were actually trying to grab them or shout at them). It's never been an issue and I've never been mistaken for police. I have lots of other stickers that I rotate on my van, GSD related, dog sports related, and Michigan related ("Great Lakes - Unsalted and Shark Free"), it's pretty obviously one of those dorky sport dog vans! I even have a new one I need to put on that says PETA People Eating Tasty Animals.
Nikon has a plain black harness that we use mainly for nosework (trial harness), flyball, and agitation work during protection and it came with the Julius K9 patches but I have ones that say NIKON, FLYBALL, and BALL JUNKIE. They also accidentally sent me one that says TURBO, lol, not really sure what that's for (if anyone knows a dog name Turbo, let me know!). I don't use any patches that would give someone the impression he's any sort of working, security, police, or service type dog. As he ages he's actually getting more outgoing towards strangers so I don't want people to think he shouldn't be interrupted or approached, generally if I have him in public I am happy if people want to meet him or pet him, otherwise I take him to train places that are more secluded. For walking around the block or going anywhere that doesn't need an additional collar or harness for training, he wears a normal 1" orange buckle collar.
For a dog who is wearing a harness or vets and shouldn't be approached, I've seen lots of patches that denote the dog is in training and should not be pet but don't say "K9". Also in some of the dog sports I do, if a dog is wearing a red bandana or has a red ribbon on his collar, that is supposed to signal that the dog needs space from other dogs and people.