My passcode was disabled

I'll start out by saying that my wife is the only other person that knows my passcode and she sad that she didn't disable it and doesn't even know how to. I was sitting at home on sunday night and noticed that my phone wasn't requiring the passcode to unlock so I checked and saw that it had been completely disabled.
The only other person to even touch my phone was my sister in law and she claimed she wanted to feel what it was like with an invisishield intalled because she has a hard case on hers. I'm thinking that she somehow disabled it because her husband has an invsishield so she should already know what it feels like. She also asked me if I had a passcode on it before I handed it to her.
So she either found a way to disable my passcode, my wife lied and actually does know how to disable the code, or I disabled it in my sleep.
I have since enabled the passcode and am quite stumped. Anyone have any ideas?

 

You definitely need the passcode to disable it and I was at the in laws when it happened so the phone wasn't reset.