Alcohol disappearing from sealed, unopened bottle?

So I bought this limited edition “rolling stones skull head vodka” box a little over a year ago, completely sealed and brand new.

After purchasing, I was heading to my holiday home and left this in one of the cupboards and have not been back for just over a year. I have just visited again and seen that although its still sealed, half of it is missing! Nobody else has had access to the home other than me and its in exactly the same place I left it. Any ideas on what has caused this?