From experience, 99% of the panic is from the journalists writing about it. The passengers, in the main, remain calm, and do the right thing.
From what I recall, it started as an unofficial fix from the CSMs, who’d just leave a mask plugged in after they used the system. Then some started putting one in every time they got on board, in case the system was used in their time off. Eventually the company realised it was a good idea. I think it was in/near the forward lower galley. I’ll ask around.
