This is a quick furrowing of the brows, while still sporting a smile, that lasts one second at the very most. When one of us sees this, we know it's time to break away for a quick conference to find out "what's up".
- Sometimes the conference isn't necessary. You kind of figure out what the frown is for. But if there's doubt, a chat is best.
FWIW, we use this method in vanilla and swing settings. It is very versatile, as it can be used for anything from "this guys being an ass", to "we need to change the subject, it's upsetting Aunt Myrtle".
This is one of those things we never set out to make our code, it just developed over time like so many things couples do to increase non-verbal communication.