I agree with Nora's parents, I also believe there was a criminal element to her disappearance. I've always thought the key to solving this mystery, is working out how Nora left the family's accommodation that night. That's something I've thought about a lot.
At first I considered an abduction to be highly improbable. The problem with an abduction scenario is, in extracting Nora, it's likely her siblings would have been woken too. For a perpetrator, that makes an abduction, in these circumstances, far too risky. Then I was considering how Nora could have been taken, without her siblings being disturbed. The most likely method, as someone has already posted, would have been to drug the children. If they were drugged, it was most likely sometime earlier in the day. Then, in the middle of the night, someone, maybe two people, entered their accommodation, probably using a stolen or copied passkey. They crept upstairs, and with all the children deeply asleep from the effects of a drug, took Nora from her bed. They then walked or carried Nora downstairs and out the front door. Before closing the door, one of the abductors opened the window, as a decoy and diversion. Then Nora was carried to the nearest road, placed in a waiting van or truck, (the noise of a vehicle was heard on the night) and driven away. That's why no trace of Nora was ever found in the vicinity, and there were no sightings of her either.