I'm a cadaver dog handler and I might be able to give some insight, maybe? I'm just going by the Wikipedia article but from the description it sounds like the dog was actually on the neighbor's property (because the neighbor gave permission for them to search there) and the dog alerted on the boundary.

There are a few problems here. One is that the dog was not a police dog, and unfortunately there have been some major issues with shoddy non-LEO cadaver dog handlers that have sort of ruined our credibility. I'm not actually familiar with the dog mentioned in this case, but I know my team has faced some huge issues after some things like a very famous (in these circles) case where a handler with a ridiculously high success rate was discovered to be planting evidence. I know a handler who is blacklisted by our local law enforcement but still manages to con desperate families into payment, and has had at least two landfills excavated because the dogs alerted on "human" remains...but actually this handler just apparently doesn't know how to train for scent discrimination and the dogs were alerting on rotting animal carcasses, and both victims were eventually found very far away from the landfills. I want to be clear here that I am not saying the handler in this case is unethical, as he probably is not (most of us aren't, it's not exactly a lucrative field), but that this is stuff that can color how law enforcement and judges see it.

Court stuff gets murky with volunteer cadaver dogs especially. My team mostly does suicide and accident victims, but we've assisted with a few murder investigations and your records have to be perfect, and even then your credibility is called into question--even if you are out there with a police officer and a warrant and your dog finds evidence.

So this volunteer/private business/whatever handler comes in and searches the neighbor's property and his dog alerts on the boundary, and he takes that to the police. Even if they assume he's trustworthy, they still can't do anything without a warrant. It seems like Flores was pretty thoroughly investigated at the beginning; asking again probably won't help. So what can they really do without a warrant?

And getting that warrant isn't necessarily going to be easy. Does the cadaver dog handler have those perfect records? Has the team (handler and dog) been proven in multiple blind tests? How many false alerts does the dog have? Has the dog been trained on scent discrimination, or is it possible that he's alerting on some discarded chicken bones from the last Flores family barbecue (this happens if you haven't trained for it!)? Are we sure the handler is trustworthy and wasn't cueing his dog to alert (this also happens, often subconsciously which is why blind tests are so important)?

So yeah, cadaver dogs are valuable tools but there's a lot of potential fallibility there. People don't seem to really understand it but both the legal precedent and the training are constantly evolving. It's a relatively new field anyway; modern canine search techniques and our study of scent really traces back to WW2. Before that there were some limited use of scent dogs to find people for a few hundred years, at least anecdotally (St. Bernards are the big ones), but otherwise scent dogs were mostly used for hunting which is pretty different in application.