As an ex-police officer I understand the role and responsibilities of a scene guard from a police officer perspective.
Were possible we would only deploy ex-police officers, ex-military and other ex-emergency services for certain low risk crime or incident scenes which need to have a visible police presence on a 24/7 basis while investigations are completed. All will have experience of working with confidential and sensitive information, experience of working in a police environment or similar and dealing with confrontation.
Preserving the integrity of the crime / incident scene, running a scene log, detailing all persons entering / leaving the scene, dealing with enquiries from the public and media.
This would free up officer time for more complex inquiries and investigations. This particular duty could be performed without a policing warrant, leaving police officers to deal with greater threats to the community.