Siri is out of control. The words seriously, Sherry, surely, etc trigger the Siri response without warning.
The newest update did away with having to say @hey” but the feature needs some fine tuning
iPhone 4s, iOS 9
As they may or may not read it here, send your feedback to the folks that work for Apple: Product Feedback - Apple
I’d probably also leave the “hey” as required, given your reported experience.