I understand and agree with the argument that she might be with a colleague/ work lunch/ etc .... you don't get it ....
It is like this ...
if someone came to me and told me that they saw my wife with a guy in, lets say, Alberton, I would be curious and suspicious. Because I know she never goes to that part of the world - ever.
Similarly if I saw Karen in Centurion, the same alarm bells went off because she never comes to this part of the world except with Mark.
It is as simple as that.