I understand it’s from people logged in to Google, but I’m getting it on ALL hits from Google, not just a percentage like it used to be. I hardly believe that 100% of people searching on Google are logged in, why is this happening in the last few weeks on all my sites?
I have only recently started running Piwik on a live site, and although a large majority of Google searches only show as “Keyword not defined”, about 15% of search terms are still visible (including searches via other search engines, although these are only about 5% of total searches so far).
It does seem a little odd that you see the details for absolutely no Google searches at all, but perhaps it may depend on your site audience? Despite the known severe privacy problems, usage of GMail, Chrome browser and Android devices has increased enormously amongst “ordinary people” in recent years, and so sadly I can well believe that there are now many people who do browse the web logged in under the ever-watchful gaze of their Sauron Google Account all day long. The advertisers and intelligence agencies must be very proud of them… 
Have you tried searching for something on your website yourself in order to check that your own searches do show up, at least?
Good news: you can now use our new Search Engine Keywords Performance plugin which lets you import your Google, Bing and Yahoo keywords into your Piwik. Learn more about the reports provided and how to import keywords on the plugin page and in the Search Keywords Monitor User Guide.