I have a week old Samsung Galaxy Grand Prime, and I noticed the same thing on the way home from shopping. I had the phone in a bag, and attributed the first time I found my phone on mute to an accident. However, I had the screen locked and the phone sitting on the bed away from anything that could result in button presses, and came back to find out missed a text message after adjusting my volume, because the phone muted itself again. I don't recall if this has happened before since I've gotten the phone, but I'll keep an eye out. Mind you this is now 2016. I went looking for this problem on threads, when I found this site, and found it worth mentioning now on the off-chance somebody else stumbles across it and has this problem, or found a solution. On that note I have discovered my new phone has a nasty penchant for writing out the word "comma" or "Kama" instead of "," during dictation!