The exact appearance varies, however, there is typically a box or apparatus at the front of the collar. Do an image search for “electric fence collar” in your favorite search engine in order to educate yourself.
Not all visible fences are constructed the same way. Had their been a visible fence, the cop may have claimed that he thought the dog would jump the fence.
The problem does not lie with the dog, which was contained in its yard.
The problem lies with the cop who shot a dog which had not left its owner’s property.
The problem lies with that cop’s fellow cops who have declared this shooting acceptable.
There is accumulating evidence that electronic fences tend to make dogs more territorially defensive because they’re restrained but others can trespass onto the dogs territory at will. This causes hyper-vigilance bordering on paranoia.
The reason I know this is my neighbor had to put down what had been a really sweet puppy that became practically a paranoid neurotic attack dog, and the only thing anyone could see for this was the electronic fence.
I have a Golden Retriever. He is very protective of his territory (our backyard) and will ferociously howl at anyone who approaches the fence. Yet, he is a sweetheart while on a leash.
I mention this because an unscrupulous breeder of Goldens had a prize-winning dog who was unpredictable and would attack though unprovoked. Because this dog was a champion, purely on looks, the dogs genes have dispersed through the breed. You must be careful when purchasing a Golden Retriever now, and check out the parents for unstable behavior.