I didn't even know how to react

Okay, I'm going to try and explain this in the simplest way possible, because no one cares about the science-y part of this, but some of it is key to the story. 

There are four types of bacteria that I grow every day for the use of my entire group. They are always labelled by my intern with the name and then I label them later with the date, so essentially there are four big test tubes of cloudy liquid with two different handwritings on them sitting in the incubator every day. On this particular day, I promised to share one of those with the handsome Belgian lad. 

I took what I needed in the morning and left the rest in the incubator for the Belgian. In the afternoon, he went looking for it and couldn't find it. Exasperated and thinking he was just being lazy, I went to look for it myself. True to his word, it was gone.

My first thought was that one of the interns had mistaken it for his boss' and used it. No big deal.

We asked him. He hadn't even touched it yet.

Damn.

The Belgian asked one of the temporary employees if he had it. He said no but offered him some of his own.

As I was marvelling over where on Earth a tube of bacteria could have wandered to, my friend commented that S was doing some work with the temp and she had bacteria. On a whim I went down the hall to see what they were using. There on the bench was a tube that clearly had my intern's handwriting followed by mine on the label.

"S, where did you get this?"

"It was in here. We grew it yesterday"

"No you didn't. That's my handwriting. This is mine. Why would you take it?" This would have been the perfect time to act like it was a mistake, apologize profusely and end the whole matter.

The Temp chimed in. "No. We grew this together yesterday. It was in here."

"Don't tell me I don't know my own handwriting!" I was so flabbergasted that I walked out. Two minutes of seething later, I had to go back and prove my point. I brought two more identically labeled tubes along to make my point.

Confronted with the evidence, they continued to insist that it had magically appeared in their incubator and they didn't know how on Earth it got there. I said, "What's done is done, but this was mine. Don't tell me I don't recognize my own handwriting. The Belgian needed this for his experiment today." And I walked out.

Still no apology. Still no reasonable explanation how something without legs walked down the hall. I don't even know how to react to this.