I'm glad you got your knife back man. Maybe you should consider using a cheapo box cutter while at work from now on. IME if you are gonna get ripped off, it'll be by someone you know. Sad but true.
I had something similar happen at work. Someone went into my shack while I was checking in a semi (down at the bottom of the yard) and cut the strap on my backpack. I too felt that sick feeling. Not because I was particularly upset about the strap being cut, but that someone I worked with felt the need to do it.
I hated that I felt I couldn't trust "my guys" out on the yard. I hated looking at people whom I thought I could trust with scrutiny. I thought I had a good rapport with them.
I'm not employed by the same company as them but we work side by side, day in and day out. Sure I catch mistakes they make but I'm never a dick about it. I'm a realist, shit happens. I don't care what the circumstances were, I just want things to be right before the customers leave my gate. In fact, only one person has ever been written up because of me in the 2 years I've been there. It was because the driver failed to secure his load before leaving the yard even after I, and another driver told him it was not safely secured.
I asked around and let them know I'd be reviewing the camera footage and despite the fact, no one ever took responsibility. The camera doesn't look inside my shack but you can clearly see anyone that would enter it. There was one guy besides myself that entered.
When I confronted him, he still denied it. I wanted to smash him. Guys a freakin weasel. I talked to his manager and basically was told that because we couldn't actually see him do it there was nothing that could be done.
It all came out in the wash though. Management told the guy they wanted to move him inside to work in the back/stock area. He didn't like that idea to much. He quit about 2 weeks after that.
The sad part is, even when you think you can trust people or you get along with them, doesn't mean they won't do something sideways for whatever reason. Human nature sucks sometimes. You gotta watch out for yourself. You are your only defense.