Evidence of an abduction are many, as John has already pointed out. The shutters and window in the children's bedroom were never opened by the McCann's as the room was just for sleeping in. When Kate went to check on the children on the 3rd of May both the shutter and window were open.
Madeleine was missing from her bed, her cuddle cat left on the bed, the toy that Kate McCann said was Madeleine's toy of comfort if she was upset. If she had woken and found the McCann's not in the apartment and she had gone looking for them then surely she was have taken her comfort toy?
Madeleine's shoes were also left behind. The patio doors were closed, the gate at the top of the steps was closed. So if Madeleine had gone out to look for her parents she would very tidily have closed the patio doors behind her and the gate at the top of the steps.
Jane Tanner sees a man carrying a child in his arms, not carrying the child against his shoulder as is the usual way of carrying a child who is asleep, especially if the child has not got any kind of covering for warmth, but in stretched out arms an awkward way of carrying. The way in which someone might carry a child who has been handed over by someone else and then the person having to hurry away not having time to switch the child onto his shoulder.
The man Jane Tanner sees is also wearing warm clothing, as Jane Tanner says he didn't look like a tourist as he wasn't dressed as a typical tourist. Strange that this man was dressed warmly on the night of the 3rd of May which was by all account a chilly night as Kate McCann stated. This man had not taken a blanket for the child to keep her warm, yet dressed warmly himself.
The Smith's saw a man carrying a child in his arms at about 10 o'clock, wearing very similar clothing and carrying a child with no covering, no blanket or coat, yet this man too was dressed warmly himself. He was spoken to but did not answer, turning his head away.
Neither of these two men have come forward to say 'it was me', everyone would have known about the missing child in the following days and the man that Jane Tanner saw and the similar looking man that the Smith family saw, yet neither of the men seen come forward to help the police eliminate them from the investigation.