Anything suggestive of foul play, danger, or illegal activity should be reported.
Any vehicle moving slowly and without lights, or following a course that appears aimless or repetitive is suspicious in any location. Particularly so in areas of schools, parks, and playgrounds.
Parked, occupied vehicles containing one or more persons are especially significant if observed at an unusual hour. They could be possible lookouts for a burglary in progress, even if the occupants don't appear to be.
Vehicles being loaded with valuables are suspicious if parked in front of a closed business or untended residence, even if the vehicle is a legitimate looking commercial unit.
Apparent business transactions conducted from a vehicle, especially around schools or parks, and if juveniles are involved, could mean possible drug sales.
Persons being forced into vehicles, especially involving juveniles or females, may mean a possible kidnapping.
The abandoned vehicle parked on your block may be a stolen car.
Continuous "repair" operations at a non-business location could mean stolen property being stripped, repainted, and otherwise altered.
Open or broken doors or windows, at a closed business, may indicate a crime in progress or already completed.
Unusual noises such as gunshots, screaming, sounds of combat, abnormally barking dogs. Anything suggestive of foul play, danger, or illegal activity should be reported.
Some of the suspicious situations described could have innocent explanations, but the police department would rather investigate a possible crime situation than be called after the fact.
Your call could save a life, prevent an injury, or stop a criminal act.