Slow moving vehicles. Burglars sometimes look for their target by driving aimlessly about, passing by repeatedly.
Parked, occupied vehicles. Burglars also will park their car and strike the home of someone they see leaving.
Vans and trucks. Be wary of a large vehicle you've never seen before parked next to a home or in a driveway. Verify that it belongs there.
Remember, trust your instincts. If you observe something that just doesn't seem right, follow through. Don't say to yourself, "It's probably nothing and besides, the police are probably too busy to check it out". Actually, police officers would much rather respond to your call and have it result in a false alarm than to miss an opportunity to prevent a crime or apprehend a criminal.