Rajendra Singh, scientist at Central Forensic Science Laboratory, told the court that door of Aarushi's room was open when the crime was committed, as bloodstains had been found on the side of the door that faces outside.
The shape and size of bloodstain found on the front side of the door determine that the murderer was inside when he shook the weapon in his hand, he said.
If he had been outside, the stains would have been of different shape and size, Singh added.
Singh was also questioned regarding the "dummy scene" enacted at the spot of the murder by CFSL and CBI officials.
He, however, could not recall if the team had carried out any sound examination during the re-enactment of the crime.
Teenager Aarushi and domestic help Hemraj were murdered in 2008 and her parents, dentists Rajesh and Nupur Talwar, are being tried for the killings.