To be fair of rain can accumulate there then I'd say it's rust. If someone crashed and caused that sort of damage there would be plenty of signs else where. I would go as far to say if that sort of damage was caused during a crash then the frame/bike would have been written off. Without fully inspecting it I couldn't say for sure. But it could be rusty patches. Whats it like for rust else where.
doesn't seem too bad for rust,although it rattles a bit when you shake it. gonna strip it back to bare metal and check it all i think,make sure there's no stress fractures.
