What probably invited the thieves was not only the garage door being open but the garage LIGHT being on, as if to beckon them, saying, "Come, steal from us!"
Because we typically close the garage door, no, the cars weren't locked. I went to bed early last night since I had to be up so early this morning, and Betsy just plumb forgot to close the garage door (not to mention turn off the garage light). No other reports of burglaries in the neighborhood--Betsy asked the policeman who came to take the report.