The police are called to a murder scene. Someone is dead in a house and the other inhabitants found them on their return from a wedding two hundred miles away. The time of death is three hours before the body was found. The police check the alibi of those who found the body. Three hours earlier they were saying goodbye to a large group of other guests at the wedding and getting into their car to drive back. I think the police could be said to have ruled them out.
The police are called to a holiday apartment because a child has disappeared. There is no evidence of forced entry and a door was left unlocked. The last people to see the child were her parents. They were at a local restaurant having a meal with 7 friends. Periodically the members of the group would leave the restaurant and return to the apartments to check on the children. One of them was seen close to the apartments early in the evening. None of the others were seen there. Restaurant staff confirmed that people left the table, but didn't see where they went and couldn't say who left and how long they were gone. As the toilets weren't in the restaurant people leaving the table could have been going there. One of the group did so, but none of the restaurant staff reported her trip. No-one was able to give an exact time when the child's mother found her missing. The resort staff were informed by another holiday maker, not by the group. the police were called 40 minutes after the disappearance was discovered. How is it possible to 'rule out' these parents and their friends?
Well obviously the have interviewed all relevant people, such as waiters etc. and come to the conclusion that what the parents and friends have said is correct.
For a start the McCann's say the police were rang at about 10 past 10, who is to say that SY have not spoken to reception and that the person who was asked to ring the police has said that in fact he didn't, that he instead got in touch with the person the OC uses in these circumstances, as they have [according to one of the nannies] had children go missing before.
All witnesses would have been interviewed and that is how they come to a conclusion.
They could also have got in touch with the waiter who served them dinner, he could confirm what time they ordered dinner etc.