There is /not/ a moral obligation to actively go looking for and judge someone as a normal individual. Rifling through another's possessions without probable cause is already wrong for actual law enforcement, let alone normal people.

I guess what I'm saying is, if in the course of normal actions I do something (like opening the web browser and the default tiles are... or the background of the desktop is) and it's obvious that this is something the individual went of the way to see / keep, that's one thing.

If it's some BS ad that probably got pulled in via automation that's another question. Once you pull in individuals politically motivated to be 'tough on crime' for re-election or future offices/career developments it's likely going to be at least a major hassle even if they're actually innocent of the intent to commit a crime.