Hi. I did go through GA several years back, think back to 2011, but didn't really see dramatic changes in traffic other than a general trend of just low organic traffic throughout. Keep in mind that it's an engineering site, so no thousands of visit per day... the keywords that are important for the site get below 1000 searcher per month (data from the days when Google Keyword Tool shared this info with us mortals).
That said, I do notice in roughly 60% of the links absolutely no regard for anchors, so some are www.domain.com/index.php, Company Name, some are Visit Site, some are Website etc. Some anchors are entire generic sentences like "your company provided great service, your entire team should be commended blah blah blah". And there are tons of backlinks from http://jennifers.tempdomainname.com...a domain that a weird animal as there's not much data on who they are, what they do and what the deal is with the domain name itself. Weird.
In all honesty, nothing in WMT or GA suggests that the site got hit by either Penguin or Panda....BUT, having a ton of links that originate from non-existing pages, pages with no thematic proximity to the client site, anchors that are as generic as "Great Service"...is it a plus to err on the side of caution and get them disavowed, or wait for a reason from Google and then do the link hygiene?