I don't think there is a large amount of blood in the home or anything too obviously disturbed based on this:
"Family members looked for Guthrie for about an hour before calling the Pima County Sheriff's Department, according to Pima County Sheriff Chris Nanos."
If family was searching in the home and surrounding area for an hour before calling police, they would have seen a large amount of blood or other obvious evidence of a homicide and called police sooner than an hour after arriving.
IMO, the sheriff's office who is calling this a crime scene has other evidence to call it a crime scene, whether that be video evidence, fingerprints, or something else that isn't as obvious as large pools of blood would be.
The Pima County Sheriff's Department has been searching for 84-year-old Nancy Guthrie since noon Sunday.
tucson.com
JMO!