Google analytics tracks your time on page, and is probably storing values to do with that when you lose focus on the page - this is used in analytics for showin

Google analytics tracks your time on page, and is probably storing values to do with that when you lose focus on the page - this is used in analytics for showing site owners engagement etc. Re the pref cookie, it could well be to track your interaction with searches, as they do for site analytics, if it changes when you leave a page, that's the most likely explanation.