There is video of John carrying a glass out of Waterfall, then Karen says he took her vodka glass with him when he got out of the car.
We don't know that he had his phone in his hand at the same time as holding the glasses. It is possible to put drinks down on the kerb, for example, while checking messages.
IMO
Isn't it also possible then that he took his glass out of the Waterfall, drank it all on the way to 34F, left and empty one in the car and took Karen's so he had another drink, henceforth, one glass?