You're leaking when your computer is on/active, with fine granularity. Pulling out reasonable estimation of your daily schedule from that data is easy.
Is that new? Back when they started implementing they were saying that the did it as privately as possible, so that google couldn't track your browsing. It seems unnessecary. You do t get hits very often, so they could just redownload the entire list when there is a hit if they feel like double checking.
Isn't reader an offline functionality?