This at the moment is quite plausible but at the same time unlikely as well.
Unless a quite skilled hacker has some grudge against you specifically, I do not see these scenarios playing out. What are the chances that a hacker will target your house's smoke alarm AND your specific car from unlocking. If the hacker is targeting the masses, im also quite certain that there will be some people who will be home when they receive the threat, therefore allowing the authorities to know about this immediately.
Now, it is hard enough as it is to get a lot of people on new technology these days, so I humbly doubt that the whole neighborhood will have many, if not, the same automated technology pieces installed in their house. In our current world where the people who have smart appliances are the minorities, is there really a business for the hackers to exploit?
There is also the issue of there being too many products that are "smart." While it is a problem in current technology because users are either windows or mac, with windows users being prone to being hacked because they are the masses and people dont do security update or are plain gullible to phishing schemes, reputable companies like Google and Amazon who are making these products are rarely hacked on their end due to strong security measures.
However, we are still miles away from the real automation that everyone dreams of, so I think we can rest assured for now.