Report suspicious people by following the 6 elements of reporting
For example,
When: April 20, 2016, time was about 11AM
Where: S Market parking lot
Who: A white male about 5' 11". Wearing a red jacket and black jeans.
What: He came to client vehicle
How: He broke the back window
Did: He stole a black bag and notebook
After incident, you must have to provide evidence. Check the CCTV or any other form of evidence and report to police officer on the evidence you have. You can report the car he/she was driving (Model and brand) so the police officer can easily identify the person of interest.