I don't really have a constructive opinion on this as I haven't reviewed the evidence available but I noticed a few people talking about lie detectors.
Absolutely no way, if I were in the McCann's position and I were innocent, would I undergo a lie detector. Polygraphs have been shown to be accurate, at best, in 96% of cases.
That 4% may as well be a gaping chasm. That's one in twenty five questions in a range of environments judged incorrectly. Say you were innocent, even if you went in all "I have nothing to hide" the lie detector may say you were lying.
Then what?
No there's a reason they don't use them in a court of law and they are only the domain of entertainment shows. To use one in these circumstances would be wrong and could lead to the victimisation of an innocent person.