I once had a young woman living next to me. She told me that the prior night someone had banged on her door, late at night. So, she felt COMPELLED to open the door! Fortunately, the guy was just odd, rather than dangerous, and just looking for a prior resident. But, it could have turned out bad.
The moral of the short story is: Install a peep-hole in your door and use it. Never feel embarassed about not opening the door.
If you can't make out who it is, don't open the door.
If you hear a knock at your door late at night, the first thing to do is to take a deep breath.
Then grab a weapon. Some will say, "Don't do that, they'll use it against you!"
That's insane. Someone who would venture into someone's house late at night, knowing full well someone is there (car is a big hint and your actions), has evil intent to begin with.
Then, grab your cell phone and carry it with you.
Always have backup batteries! And backup of that. Be sure they are in your nightstand by your bed and have batteries in other places, as well.
I will give you many other tips, but I don't want to overwhelm you. That will serve little purpose.
If the person hovers around, call 911. Always be one step ahead of trouble.