I have never gotten to that point yet. I did ask an FA once about a pax in F being in the bathroom since I saw an empty F seat. Once she said yes, I continued to my Y seat. But I have seen pax ask the FA to go speak with the GA before (for various reasons). I would not be seated at this point - I check F seats as I board (remember, the GA said F "boarded full") so every seat in F should be taken. Doesn't work with 757's if you board through 2L. But most times it works. I am one to hang out at the gate if there is any chance of an UG.