Perhaps it was the weather during the seagull along with being near a salty beach environment? I started taking my 14 yr old Diamondback mt. bike down near there on vacation in recent years. It seemed to really start to show rust after taking it there. Clean it off, lube it up, and see if it happens when you are back home. You say you already have mileage on the bike, but just saw it after the seagull. Maybe it was just that.