If you see this look on the face of a woman you’re seeing, the relationship is as good as dead. And there’s not a damn thing you can do about it except pack your bags. I know it well. It’s the look of a lady who hasn’t yet said “we need to talk” but is definitely working her way up to that. Except there’s no need. You’re history and that’s that.
Guys don’t use this look because they don’t tend to call things off (although sometimes they do). They just keep nodding and smiling as they try to figure if it makes sense to start seeing someone on the side, or if they should try to be a bit more constructive and conciliatory.