Whilst escaping the sewers you will come across a safe that needs a combination to be opened. Nearby you find a note saying “look for the whiskey”. If you jump over the safe towards the back and move some of the whiskey bottles in the cupboard you will see the combination 451:
At the end of this guide there is a video with a visual version of this guide.
I’m not sure if it is the same for everyone, I imagine it is. Simply go back to the safe, change the combination and the safe will open.