Who is The Culprit?

A massive discussion takes place in the chowki in regards to Darpok's Shaving cream problem. Gulgule tells Chandramukhi that Darpok might be suffering from a sleep problem. Chandramukhi orders Gopi, Gulgule and Billu to spend a night in Darpok's house to check who is applying cream on his face but they were shocked to see the cream on Darpok's face in spite of their tight security. How did it happen?