From the Guidelines

The air will most likely accumulate in the retrocolic space if a perforation occurs in the duodenum.

The duodenum is a retroperitoneal organ, meaning it is located behind the peritoneum, a layer of tissue that lines the abdominal cavity.
When a perforation occurs in the duodenum, air can escape into the surrounding retroperitoneal space.
The retrocolic space is a part of the retroperitoneal space located behind the colon, and it is a common site for air accumulation in cases of duodenal perforation.

According to the clinical practice update on endoscopic management of perforations in the gastrointestinal tract 1, type 2 periampullary perforations, which occur in the retroperitoneal space, can be subtle and easily missed.
This suggests that the air from a duodenal perforation can accumulate in the retroperitoneal space, which includes the retrocolic space, rather than freely flowing into the peritoneal cavity or Morrison's pouch.

Understanding the anatomy and pathophysiology of duodenal perforation is crucial for accurate diagnosis and treatment.
In cases of suspected duodenal perforation, imaging studies such as computed tomography scans should be carefully evaluated for signs of air accumulation in the retrocolic space.

From the Research

If a perforation occurs in the duodenum, air can accumulate in various spaces, depending on the location and extent of the perforation.
According to the study 2, air can extend into the right perirenal space after duodenal perforation, which is located behind the colon and kidney.
The study 3 also mentions that air can be visible in Morrison's pouch, which is the hepatorenal recess, in cases of perforated duodenal ulcer.
However, there is no direct evidence to suggest that air will accumulate in the retrocolic space, which is behind the colon, as a primary location in case of duodenal perforation.
The lesser sac, also known as the peritoneal cavity, is not mentioned as a primary location for air accumulation in case of duodenal perforation in the provided studies.
Morrison's pouch, also known as the hepatorenal recess, is a possible location for air accumulation, as mentioned in the study 3, which reported a case of confined duodenal ulcer perforation with air visible in the fissure for ligamentum teres and a small amount of free fluid in Morrison's pouch.

Right perirenal space: as mentioned in the study 2
Morrison's pouch (hepatorenal recess): as mentioned in the study 3
Other possible locations, such as the right anterior pararenal space, right posterior pararenal space, and bare area of the liver, are mentioned in the study 2, but are not directly related to the question.