Simply observing activities is not a crime. Doing so to facilitate a criminal act COULD be a crime, depending on the specific actions, intended crime, and statutory wording. However, proving that intent is going to be very difficult (absent a confession) until the intended crime is actually attempted. An officer may be able to articulate enough reasonable suspicion to lawfully detain and question the suspicious person, but (again absent a confession) it is unlikely that any further action would result.

