Work metal detector went off for alcohol in cardboard

I don’t have I car so I have to carry my personal belongings with me. I ran across the street for my lunch and picked up 2 beatboxes the alcoholic beverage for later. I get off usually after 12, so knowing that I went ahead and got my things. Both times it went off, when I came back from the store and when I left work. Those are the only things I bought so I know it’s the drinks. I did some research and the only thing I can come up with, is the detector is an enhanced one for detecting alcohol, and unless of course there’s metal in these drinks. It’s made out of cardboard and plastic, and did a brief examination of the bottles and it didn’t look like there was any metal on them.

Any possible explanation?