The likelihood of a false alert is hard to say, as it's highly dependent on the individual dog and handler, as well as the type of search and a bunch of other factors. Generally, though, cadaver dogs are pretty accurate, particularly when alerting on actual human remains (as opposed to trace evidence).

When dogs give a false alert, it's usually handler error. Either a cued alert (which can happen by accident or intentionally, but IME is usually an accident), or some dogs use an untrained indication, which has some benefits but also has the potential to be misinterpreted by an overeager handler.

HR is a pretty distinct scent and there's nothing really that mimics it. Some handlers don't spend enough time teaching their dogs the difference between HR and animal remains, so you could potentially get a false alert if there's not enough proofing done, but that's rare enough that I wouldn't count on it.

There's also the potential for a correct alert that's unrelated to the case, but that's usually more an issue indoors in places with a high level of contamination risk (for example, someone cuts themselves slicing vegetables and bleeds all over the kitchen, then next week their house gets searched, the dogs might alert on trace amounts of blood in the kitchen even if there was never a body there). I don't think I've ever seen that happen in an outdoor search, though.

Sorry that probably doesn't really help much in figuring out whether the K9 evidence in this case is accurate. It's hard to really gauge based on vague news sources and the like, though. Sometimes there'll be details that hint one way or the other, but usually the information easily available is so vague that it's really hard to tell, especially because reporting about search dogs tends to be of questionable accuracy at the best of times just because people don't really understand them.