Conspiracy theories excite the imagination, usually because there are untidy elements in the known facts or the official version (JFK: the fact that Oswald was silenced; was he that good a shot; the Zagruber video; ... but Oswald did hit him with at least one bullet. Diana: the disinformation about the extent of her injuries; the delay in releasing the tragic news, to give
Charles time and privacy; the distraction of the paparazzi; ... but car accidents is not how assassins work). Usually, and maybe boringly, the official version turns out to be the most likely explanation, however unsatisfactory.
In my opinion, the McCanns were guilty only of an unthinkably lax child-minding "drill" and will probably never forgive themselves. It's surprisingly easy, even now, to wander through hospitals or hotels without any security check. The chances are that this "drill" (such as it was) had been watched before by deliberate child-snatchers, or else a burglar was disturbed by Madelaine. There had been previous intruder incidents in the wider area, which the local police played down maybe to avoid alarm and later to reduce criticism of their sloppy investigation. Like Lord Lucan and Jill Dando it may well remain a mystery.