Are these sentences OK?
You left the door of the house open.
You left the door of the house unlocked.
Someone could come in and steal something.

Question

Answer

All these sentences are ok, but it sounds more natural to say "You left the door *to the house open/unlocked." 
Or simply, "You left the door open" or "You left the door unlocked" (the house is understood)

Answer

The grammar is correct, but it would be more natural to say:
You left the house door open.
You left the house door unlocked.

Answer

Yes

Answer

Perfect