Nexus 6 goes from vibrate to sound profile

[dakid2k6]

Question, if anybody has this same issue. My n6 is on 6.0.1 and stock rom. Just unlocked bootloader, we'll here's the issue. I keep my phone on vibrate but it's randomly goes from that to very low volume. While at work it should be vibrate but at times goes to sound. I don't do anything. Even happens overnight while laying on my night stand. I have done factory resets and still have the issue. Any input