As you know, with most hotels, you have 24-48 hour prior cancellation policies. So if you find a better price between now and your hotel stay, you can likely cancel your reservation and rebook for the better price. Google Now is able to read your emails, find your reservations and likely through Google Hotel Finder, scan for price changes. It can then alert you as it finds lower cost pricing for the same hotel reservation you placed. Get the full story at SEO Roundtable and Google+