This one is not hard, but it is fun. I am going to make it more purposefully vague than normal, because it would be way too easy to identify them otherwise. It is still easy though.
About two weeks ago at Bungalow 8 in London, there was a table of three which consisted of a dropping like a stone actress, her girlfriend?, and the brother of the girlfriend. Unlike most tales of powdery substances in a club, this one didn't involve the bathroom. Instead, whoever was running the club that evening decided that the two women could use the room marked staff every 20-30 minutes for their activity. It does make it much easier if you are provided a space other than the back of a toilet or the bathroom counter. For the record, the brother didn't touch the stuff.