Her original story was she barely saw anything.
That morphed over time to several increasingly detailed (and different!) sketches, a description of his hair, his clothing, his nationality, the childs pajamas (she was really wearing the same sort as Madeleine? Really?), the fact that she positively ID'd Robert Murat as being that man. :liar:
I think that when someone wishes to dispose of an inconvenient body, and they have functioning brain cells as the McCann do, they look for logical cover ups.
They'd all seen parents walking to and from the crèche with sleeping children. (Maybe they could've decided to use the service too, but no...

) A bunch of yuppies stuck with a body and no car. Of course they're going to get it out of the apartment, so of course, they've got to carry it out.
All the best liars know that when creating a lie it is best to stick to a kernel of truth.
The kernel here is that at some point Tanner saw a man carrying his daughter home just as she described. Heck they probably all did, several times. Apparently it is a common sight in PDL.
Was it on the evening of 7 May? Who knows. The guy himself probably has no idea. She could've seen him the first night, the night before, any time really.
A small kernel of truth, such as a casual sighting of a couple of parents carrying their sleeping children through the streets, can easily be morphed into something convenient.
I would suggest that Tanners ever improving memory and description was actually elaboration of something she actually did see, sometime.
Gerry carried M away because they all knew it was a common sight and could easily be explained or hopefully, barely noticed in the first place.
:twocents:
ETA: I used to have no clue when she died, I even considered the "days before" scenario. Now I firmly believe she died early on in the apartment, probably just before the 5 minute/30 second Payne scenario.
Hopefully, not during. :sick:
Which is looking increasingly likely in my humble opinion.
Poor Madeleine.