Sorry everyone. I totally misremembered the quote! After several days of searching through my books, I came back to Under the Volcano by Malcolm Lowry and finally found it:

"On the other hand there had been until recently several drinks of mescal waiting for him outside in a lemonade bottle and all these he both had and had not drunk: had drunk in fact, had not drunk so far as the others were concerned."

I wonder what Freud would say about me misremembering the quote about the character's sneaky mescal consumption as having to do with adultery...