I agree with challenging the knock, My niece just last week had a person knock a couple times and did not answer the door, seen him leave and then went to the back room and saw someone walk by her window and try to open the back door. She ran to the bathroom and called 911. Police did a good search but and found nothing.
make darn sure anyone challenging front door knocks use a voice that means business. otherwise it'd be better to install a video camera trained on front door. setting off a loud alarm horn would do it ... bad folks are not sticking around, too many other easy targets!
there's a sticker near my front door that reads:
Nothing inside is worth your Life!