Siri does feature a back button in iOS 7 for individual search results, however, there is no back button once you leave Siri to open a different app. For example, if you ask Siri to give you a list of local pizza restaurants you can use the back button to return to the original Siri results from the "Details" page of any individual business. However, there is no back button if you use Siri to access the Maps app or Yelp app for directions or reviews. This doesn't mean you can't access Siri's original results you just have to do it in a roundabout way. Here's how:
1. Hold down your Home button to evoke Siri once you're done checking the Maps app or Yelp reviews.
2. Instead of asking Siri the same question a second or third time, swipe down when you see the black Siri screen. This will allow you to scroll up and see your original search results.