Help Notification sounds won't stay off

Hello everybody! Title says it all - I'll mute all sounds including notifications, and for a little while (about a half hour) they don't make a peep. Then, suddenly and seemingly for no reason, they just turn themselves back on, so to speak.

I haven't been able to reproduce the issue, so I'm not sure what, if anything, is related to triggering it. The closest bet I have is that it seems to happen after I unlock the phone and view a message.

I know this sounds like I'm unlocking the phone, turning the sound back on and locking it, but I'm not. It happens mostly when I'm at work, and I just take the phone out to view a text and lock it immediately after viewing.

I'm very conscious of it obviously because it bugs the crap out of me (and gets me in trouble at work), so I'm 100% sure I'm not turning the sound back on or hitting any buttons.