The Shitter, Exposed

"One of the pleasures of the show is being able to enjoy the details of the paintings, particularly the small figures in the background. This has been a game for centuries: in 1604 the Dutch art historian Karel van Mander recorded that Patinir was known as 'The Shitter', since his trademark was said to be the tiny figure of a defecating man, hidden in the composition. However, the exhibition suggests that this incongruous Bosch-like man appears in only three works (all once owned by Philip II)."

From Here

Also, I might add, impossibly hard to find on the interwebs.