I see a lot of used cars in Taiwan with brake rotors basically totally rusted out other than the area where the brake pads make contact. Assuming a 10 year old car, is that just a byproduct of driving in the rain often, or is it a sign of flood damage? Or perhaps inconclusive either way?
For example:
Brake rotors will rust if they sit. I’ve had mine start to rust within a few days of sitting (in the States). Nothing to worry about, it’s normal.
If you have reason to believe there was flood damage, your best bet is to look in cracks & crevices to see if there are any deposits of sand, etc. This isn’t going to be definitive, as any debris in the air can find a resting place, but if you look under the hood and in the trunk (moving the carpet), you may see where dirty water was.
Very cool, thanks for the informative response. 
I’ll keep an eye out for sand vs. normal “dirt” in the upper crevices, that makes sense!
One other quick question - are cars with digital odometers just as easy to hack/roll back as non-digital odometers? I believe I’ve seen a device that can do it for about $5,000 so I’m assuming everyone even digital odometers can’t be trusted here?
word is the digital speedo cant be rolled back but some people put in a replacement unit claiming the original is busted.
rusty rotors? normal because the rotors are made of steel and steel rusts.
I third that, rusty discs are absolutely not a sign of flood damage.
I second that, I’m sure some people can roll digital speedos back, but it’s probably easier to just change the part to one from a smashed car with a lot less Ks on it. Or just disconnect the speedo feed like taxi drivers are reputed to do so the miles never accumulate in the first place.
Yes, they can easily be hacked on most popular cars, and are commonly set back on cars with over 100,000km by car salesmen.
I don’t know of any car where the EEPROM storing mileage data is integrated with the instrument cluster. The EEPROM chip is usually somewhere deep within the electronics system and some cars (such as Jaguars) have several chips in hard-to-reach places to try and foil hackers (which it doesn’t). Swapping the instrument cluster will have no effect, but many vehicles will allow the mileage to be set through the OBD diagnostic port. It’s not something you can do at home but for someone with the right diagnostics hardware + software it’s very quick and easy.