When I was a teenager, our dog was on heat once, and there was a scrawny old dog that used to lie on our step all day, every day. Whenever we opened the door he'd sit up, looking hopeful, with an "Is she in?" look in his eye.
I said to the family, "There's no way that scrawny mutt is getting his paws on our Goldie." Anyway, one day Goldie escaped, not for the first time. She came back later (sometimes it was next day) looking very sheepish.
As she was on heat, we thought it could be any dog who'd had his way with her and not necessarily the dog on the step. When she had her pups, she had five really cute ones and one who was the absolute spitting image of the scrawny old mutt who'd patiently waited and waited before seizing his opportunity.