Google is turning app developers loose on Google Now, but search data is still finding its way into the feed in new ways as well. If you haven't already, you'll soon start to see cards that tell you when a product you've searched for goes on sale. Google previously did this with flight data, but now it seems to work with all kinds of stuff.
The flight price tracking was added last year as Google ramped up its flight information in the knowledge graph. This new pricing data appears to be based on your search history too. If you search for a specific product a few times, Google assumes you're interested in buying it. So if the list price drops on a page Google indexes, it can notify you.
It's not clear how or if Google differentiates between an "I want this" search and "I bought this thing and it's broken and I need help" kind of search. Presumably clicking on shopping links in the search results as opposed to support forums is a relevant signal. Either way, enjoy your savings.
- +Chris Livingston