If you have an app with app-indexing, Google is looking for some testers, according to John Mueller.  It needs to be an Android app and you must have Google’s App-Indexing search features enabled to do so.

It does require an NDA to participate, which means you will likely see some pretty interesting things, but nothing you are allowed to talk about.

Signup for Google App-Indexing preview features

We’re working on new things for sites & apps that use App-Indexing, and we’d love your feedback. If you have an Android app, have set up App-Indexing ( http://g.co/appindexing ), if you’d be ok with signing an NDA, and if you’re keen on trying something out and giving us feedback, feel free to sign up here.

This could be a very interesting opportunity for app developers to sign up for.