This seems to be a major design flaw. I'm a relatively new user and had my phone on "silent" the other day (mash power button --> choose 'vibrate'). Actually, I was in church and going through Twitter (don't judge!). Went to the home screen and accidentally hit something and my 'siri' came up and said something like "I can't find the pictures you're looking for" (or something like that) Jeez, it was truly appalling when everyone looked over. That is a truly terrible oversight.
I could not recreate that exact quote, but I noticed that even when on 'silent' if you hit the microphone in the G search bar it bongs and then if you remain silent she will say, "if you just tried to say something, I didn't understand". Also, if you happen to open the browser to a page with an 'auto-play' ad, the volume will play. This is just terrible and just about makes using your phone during inappropriate times impossible. (and yes, I understand the paradox and 'unethical' nature of the last sentence, but I don't need my phone to attempt to control my supposed obnoxious behavior). This has GOT to be a problem for others, no...or does no one else look at their phone during meetings and such?