Except for C Mitchell telling prime time on RTE that the McCann's were firmly of the view of
"that somebody got into the apartment and took Madeleine out the window as their means of escape, and to do that they did not necessarily have to tamper with anything. They got out of the window fairly easily."
All on the web if one wishes to find the cite.
I very much doubt if at that time with all the overwhelming grief they were going thru', they analysed the situation in any depth.
It seems to me that they fell into the trap that was set.
Because the window was open, they just thought that she must have been taken thru there. Later on, it is my bet that the Mccanns realised just how difficult / impossibnle it would have been for an abductor to leave thru that narrow window carrying Madeleine ...
... and leave no fibres or marks on the window, or cill, at all.
They made a simple mistake. One, imo, that they were led into