Here's an explanation of the death scent on cuddle cat. Kate says "Maybe it happened when the toy went to work with me." Through the lens of "she's guilty", it sounds like an idiotic lie. But through the lens of "she's innocent", it sounds like this:
You take your child to preschool in the morning. If your preschool is like most, "favorite" toys aren't allowed there (it can cause conflict with other kids, and it might get lost, dirty, or covered-with-paint). So ... your child takes her favorite toy to the door, but you must confiscate it on saying goodbye. The toy goes back to the car with you.
Toy spends the day inside the car in the parking lot at work, while you are busy getting your hands (or more likely your clothing) exposed to death scent. In the afternoon you return to the car, and in the process of doing something in the car, you pick up the toy, maybe put it in your lap or otherwise let it brush against you. The scent has now been transferred.
I believe this scenario is very well within the scope of "I took the toy to work with me" because in fact the toy DID go to work. The fact that it didn't go all the way inside the building would be obvious to someone who feels innocent, and to any listener who believes the person to be innocent. It only takes on the ominous tone of lieing idiocy when the listener is already convinced the person is guilty.
Big rule in sleuthing: Don't get fooled by your own assumptions.
If this was an inside job, it was orchestrated by one person, someone who is confident, a good talker, who lies glibly. Kate doesn't strike me as a particularly good or glib talker. She says a lot of boneheaded self-incriminating things, in fact. A good liar/murderer knows how to shift attention subtlely to someone else, NOT pile it all on top of herself!
Murder statistics tell us the person is most likely male. That would not be Kate.
The person had 1.5 hours to do something (8:30 pm to 10 pm) after Madeleine was last seen by both parents, and before she was discovered missing by Kate. Kate never left the table, one thing the Tapas 9 and the wait staff agree on. If Kate & Gerry knew Madeleine was dead and out of the apartment at 8:30 pm, it's ridiculous to assume they'd wait 1.5 hours before springing the trap.
Finally, the person who did this needed to put on a show afterward to impress someone that he was innocent. Someone close to him whose respect & devotion he craves and who is, or was, clueless.
The person's motive? Could be lots of things. Could've been premeditated out of jealousy, annoyance, or even for money. Could've been an accident that happened when he checked on the children and Madeleine made a scene. She cried, begged him not to leave her, stood up on the bed and lunged for him. He was annoyed, took a step back and let her fall. She hit her head on the corner of the bedside table.
Now he's in a pickle. He convinced Kate to accept these cheap childcare arrangements, had assured her there was nothing to worry about, and had promised to check often. Now he up and kills one of them. Oops my bad. He knew he was in it deep with her, if she found out. So because he is who he is ("can do", life-and-death, cardiologist, friends in high places), he took a bold step. He hid Madeleine himself or confided in one person who although he had misgivings, helped him remove the body. That person, being a doctor himself, had access to the local hospital and some kinf of cold storage.
Maybe he intended all along to tell Kate. But as things developed he found himself deeper and deeper in lies, and it quickly became impossible. He's now trapped in the lie and has probably convinced himself it all never happened. In any case he's made amends x 100 by visiting George Bush and the Pope and writing that silly blog, and all.
Meanwhile he's doing his best to keep Kate off balance, to keep suspicion and public attention on Kate.
I think this is a great theory, and does an excellent job of accounting for the key facts. Problem is, it's very Hitchcockian and really over the top. As are most of the spooky ghost-story meanderings you read on these forums. People just love getting the shivers over a good body-snatching, don't they? Oooh scarey...
In reality, the more I think about this case, and the more I hear what's coming from the "they're guilty" side, the more I'm inclined to believe they're not guilty -- either of them -- and it was, sadly, a stranger abduction.