People sitting in their cars

In the morning when I am walking the dog, I see people sitting in their cars. Sometimes the engine is running but not always.
I see them from blocks away and they are still there when I pass them and go a few more blocks.
What the heck are the doing just sitting there? Makes me think of crime shows when someone is staking out a property. A detective or something, but are there that many detectives?
Maybe its like X boyfriends spying..