Kickout flashing
Required, but often missed
Kickout flashing is a piece of flashing material of a specific size, installed where the edge of a roof intersects with a vertical wall, as seen in the image below. The purpose of the kickout flashing is to divert water away from the vulnerable siding/roof connection point and ideally into a gutter.
Kickout flashing has technically been a requirement in the IRC building code since 2009, but it wasn't until it was worded more clearly in the 2012 IRC version that some builders started to take notice. Despite this, it is very common to have missing kickout flashing even on houses constructed after 2012.
The below video is on my own house, built in 2011, in that time where many builders were still mostly unaware of the code requirement. My moisture meter is maxed out in this location. The house does have a house wrap installed. It is of the perforated variety though, and cannot be expected to be watertight for long periods of time. It is more likely that the moisture getting behind the house wrap is occurring at the transition between the wall and the roof, where even the best house wrap taping job stands little chance against water's ability to find a way in.
So, can missing kickout flashing be added with existing shingles and siding? It may be possible to retrofit an existing installation, but the complexity and cost will depend on the existing cladding material.
To sum things up, a properly installed kickout flashing can save thousands of dollars in damages to wall assemblies. Getting a quality home inspection from a company like Foresight Home Inspection can help identify issues prior to purchase so that you can make an informed decision.