Quando Snape aparece na Casa dos Gritos, ele tem isso a dizer para a festa:
Snape was slightly breathless, but his face was full of suppressed triumph. “You’re wondering, perhaps, how I knew you were here?” he said, his eyes glittering. “I’ve just been to your office, Lupin. You forgot to take your potion tonight, so I took a gobletful along. And very lucky I did... lucky for me, I mean. Lying on your desk was a certain map. One glance at it told me all I needed to know. I saw you running along this passageway and out of sight.”
Parece que Snape encontrou o cálice sentado em sua mesa, prestes a ficar bêbado, e Lupin também estava inspecionando o mapa ao mesmo tempo. (Se o mapa estava inativo, é improvável que Snape pudesse ativá-lo.)
Mais tarde, Lupin explica a poção de Wolfsbane para Hermione:
“The potion that Professor Snape has been making for me is a very recent discovery. It makes me safe, you see. As long as I take it in the week, preceding the full moon, I keep my mind when I transform… I’m able to curl up in my office, a harmless wolf, and wait for the moon to wane again.”
No entusiasmo e alvoroço de ver Sirius e Peter no mapa (especialmente considerando que Peter estava morto), ele provavelmente deixou a poção (e o mapa) como ele queria ajudar Sirius e / ou Harry, Ron e Hermione.