Does site search search SEO metadata, specifically keywords?
No. Site search looks at page content, and does not look at the SEO Keywords (or any other SEO metadata).
In terms of industry best practices, it is becoming less and less common to search the SEO keywords field due to bad behavior by site admins (particularly keyword stuffing, i.e. loading up the Keywords tag with irrelevant-but-popular buzzwords to try and increase hits). Google's PageRank algorithm has not included SEO metadata at all for quite some time, just for this reason.