LG G3 :: Why Does Phone Randomly Vibrate Sometimes

I just got an LG G3 last Friday and I've noticed twice now that it has randomly vibrated in succession- with no notifications. It doesn't happen often enough to diagnose the problem myself, so I tried to find something on it but nothing worked. My phone is always on 'Vibrate', but the problem is that no notifications show up. The last two times it has done this, it's been on low-ish battery (30s-40s), but not low enough to get the battery saver notification. I also know it isn't "phantom vibrating" or whatever, because I can hear it vibrate on my desk (out of pocket).