Rust usually does not appear overnight. It often starts from a small stone chip, a damaged edge or a moisture-trapping area that goes unnoticed for too long.
In northern climates, the body and underbody take a lot of stress from salt, moisture and temperature swings. Regular washing, especially after winter and in spring, helps remove contamination that speeds up corrosion.
If you notice a small paint defect or the beginning of rust, it is worth fixing early. The longer it is left alone, the larger the affected area and the larger the cost tend to become.
It is also useful to record rust protection, underbody washing or bodywork in the service history. That creates a clearer picture of the car's condition and helps plan care with a longer view.