I don't believe the parents did it.
There is no evidence against them. Yes, I know about the dogs - but it doesn't add up. For starters, the house wasn't actually sealed off for forensics until way too long. There were numerous people in and out of there and the hotel even hired out the house to new occupants before the police sealed it off for forensics - that's fucking remarkable and it destroys any credibility of the dogs results.
The dog was getting readings absolutely everywhere. Flower bed outside, behind a couch, etc, etc. What, they killed or found their daughter dead and just methodically moved her all over the house and even outside within a few minutes?
The dog got a positive in a rental car they hired 25 days after the murder. That means they managed to hide the body whilst under 24/7 attention for 25 days and successfully stash it in a car and ditch it.
I think the dog was getting false readings.
Aside from the dog, the parents don't have anything against them. In fact - the facts don't look like the parents did it.
Think about the fact that they let a family friend of theirs check on their children just a few minutes (I believe it was ten minutes) before they noticed her missing and raised the alarm. If Madeline was already dead, only an insane risk taker would allow a friend to enter and maybe blow their cover. So, if they did find her dead and hide the body, that means they would have had to do it after the friend went in to check, but that only leaves them with a ten minute window. That is not enough time to discover your child dead and hide her body. Even if it was - that would make the mother the murderer or body hider - not the father.
I just don't believe it was them.
There is more than enough reason to believe it was a predator:
The hotel/resort was busy. There were hundreds of people around.
The resort was in eye sight of the house they were staying in. This means a potential predator would have probably seen the parents and friends doing routine checks on their children, which is an obvious sign that their children are alone in there.
Kate (Madeleine's mother) left a note in the guest book at the front desk of the restaurant blatantly saying that they want this table reserved at 8:30 every night and their kids would be asleep in their apartment. Hundreds of people probably saw this. A potential predator all of a sudden as literally all the information they need.
Every single time someone checked on the McCann children, they noticed the door was open. Even the family friend said it was open when he checked. The predator was probably waltzing in and out of there whilst he was certain he had a few minutes before anyone else came and checked.
I realise that the house had no sign of forced entry, but it didn't have to. I realise there were no prints, but they probably wore gloves.