Currently the standard Marketing Channel implementation for Paid and Natural Search is to match based on Adobe's Search Detection rules.
However for one client (UK based retailer) it turns out that this misses out quite a lot of search engines e.g. portal based ones powered by Google such as Virgin Media's search etc. This means on the marketing channels report that search traffic is being bucketed under referring domains.
There is a workaround to this: add PPC campaign codes and search engines yourself as additional processing rules (for paid and natural search respectively). However it would be better if there could be more search engine coverage on Adobe's list.