I had a friend who said hello to someone she knew as they passed her.
The husband didn't know the woman, but sort of awkwardly smiled at her.
Friend was absolutely convinced that something wasn't right.
They went away for the weekend, and my friend was told that she was being ridiculous, as he had done nothing at all to arouse suspicion, and she was causing an atmosphere in the home of the people they were staying with (who thought she was mad!)
They did split up a couple of years later, and guess what finally came to light?
He had been having an affair the whole time with the suspected person.