"Hey Siri" activates Siri even when not plugged in to power

Hello iFriends

I have an odd thing that started back in some update for iOS 8 and on to iOS 9. "Hey Siri" being the voice activated command that summons the all knowing Siri (similar to summoning Shenron but by voice) is supposed to work only when the iDevice is plugged in to its power source. Well, since that iOS 8 update, it's on all the time and it works whether or not I'm plugged in to power. The iPhone 5s was purchased brand new upon release (preordered as always with iPhone, but only the second generation "s" model, never have/will first generation). "Hey Siri" worked as it was supposed to until some later update, then all of a sudden, I was watching a movie one night and heard the famous "dudink" that Siri was activated. My phone wasn't plugged in to power so even if I said "Hey Siri", it shouldn't activate. Sometime later, I re-watched the movie and heard the tone again! Neither time was I plugged in to power (I have a very particular "plug in" procedure that I've followed for years now). I figured out the moment in the movie that activates Siri and realized whatever they were saying (haven't watched in a while), somewhere in the middle of them talking, the sounds "hey siri" did appear. I don't use Siri at all, so by default, I don't use "Hey Siri". I'm just curious if you know of a hidden setting that may have activated it all the time? I tried using google but all I see are jailbreaks to activate it. None of which I've done.

Thanks

Minhcalla Ink