During a game, when an event takes place, you usually have bandeau crossing the screen above the crosshair by default. This bandeau is normally grey with some text inside.
On the map Chapel, attacking side, first objective (repair, or right after) a similar bandeau appeared excepted it was red with written only ‘Gas’ or ‘Gaz’. I can’t remember the spelling. I was then below the MG bridge or nearby.
I believe this is related to a message when you cross a gas area, although I never pay attention such a bandeau was existing. I was far from any gas anyway (Chapel map).
It happened only once to me, and I didn’t have time to take a screen, but if you grep your code, you ll surely find out what I mean.
Maybe someone will succeed to take a screen of it.