Once you find out who has is taking your coffee, whether by ink on the handle or whatever, don't do anything for a few days.
Then at the bottom of the coffee pot, on the inside to be revealed as the coffee gets poured, put a photo of their first born son.
Not one taken from their desk, but shot from across the street of their house.
Then at the bottom of the coffee pot, on the inside to be revealed as the coffee gets poured, put a photo of their first born son.
Not one taken from their desk, but shot from across the street of their house.