She was designed to be outside at a Crosswalk 24/7, She's been outside during heavy rain. They probably would have known if she wasn't water proofed before hand
@Granitefish
Well, here she is in the very first chapter using an umbrella, so at the very least, she was programmed with the concept of rain being 'not good' in some way, whether to mimic humans coming down with the Deadly Japanese ColdTM or unnecessarily exposing robotic joints to the more extreme elements of nature.
@Diamona I didn't notice that until I read your comment. So that'd be a brother (HSS01 BATO) and a sister (___2F name off-panel). I'm guessing here that the F designated female, since Fuku/Shingou is female herself, and the F is missing from HSS01.
It's also possible that those were previous development models that failed and were scrapped. Somehow I doubt that'll be how it works though.