Which of the following is a crime scene 'do'?

Noting original conditions upon arrival is a critical practice in crime scene analysis. This action ensures that the state of the scene is documented as it was found, preserving vital information that could be valuable for subsequent investigations and legal proceedings. This initial assessment allows investigators to establish context, including the layout, any potential evidence, and the circumstances surrounding the crime. Accurate documentation at this stage can help in reconstructing events, establishing timelines, and ensuring that the evidence collected later is placed within the correct context of the crime scene.

This practice contrasts significantly with some of the other options, which could compromise the investigation. For instance, touching evidence can contaminate it, which could hinder judicial processes. Allowing unrestricted access to the scene can lead to evidence being lost, altered, or contaminated by unauthorized individuals. Following the routes the suspect took could be valuable, but it is not considered a fundamental principle like preserving the original conditions of a crime scene. Thus, noting the original conditions is vital for maintaining the integrity of the crime scene investigation.