Master of the craft, Dame Agatha, has this to say about ‘what one sees’. Good advice for the puzzle mystery writer.
“What you have to account for is if one person did see something why didn’t that person say so? … Possibility one. The person who saw it didn’t realize what they had seen. That would mean, of course, that it would have to be rather a stupid person. some, let us say, who can use their eyes but not their brain. … It might have been a person whose action in putting something in a glass was natural. It would be quite possible, audacious but possible, for someone to pick up that glass which as soon as it was in his hand or her hand, of course, would be assumed to be his or her own drink and to add whatever was added quite openly. In that case you see, people wouldn’t think twice…it would be a gamble, a risk, but it could happen…Possibility three…Somebody saw what happened and held their tongue deliberately … if so, it’s a very dangerous thing to do. – Agatha Christie (Jane Marple)