Malicius, I don't know what your information is on this one, but as I understand it these dogs aren't nearly as good as they're cracked up to be. Drug-search dogs have been shown to be pretty damn good at the Clever Hans trick more than actually smelling drugs. And I understand this "cadaver" thing is even more hit and miss. The dogs could have been reacting to the suspicions of their handlers.
It seems to have been the alleged reactions of the dogs that started or reinforced the idea that the McCanns had sedated Madeleine (by the way, she was never called Maddie or Maddy - it seems very disrespectful to shorten her name when she may be dead, to something she was never called in life). But I don't believe there was ever any evidence they did that.
I'm with the posters who give credence to the idea that medical professionals might do this. I don't know what Benadryl is, but I'd put money on temazepam being the drug of choice. I did meet a Dutch doctor once who told me he sometimes gave his year-old baby a bit of a nitrazepam tablet to make him sleep, and temazepam is the current version of that. It's effective, and it's very difficult to overdose with it.
However, there's no evidence at all the McCanns actually did this. And the idea they came home and found her dead, then decided to dispose of the body in case traces of benzodiazepine were found at post mortem - well, I'd need some actual evidence before I started thinking that, and as far as I can see there isn't any.
Rolfe.