Buttercup mentioned that her brother got home around lunchtime but Lorn, you said that you got home around 3:00 and that your son got there 10 minutes before you. Could your son have come home, left and come home again? Maybe the cat got out at that point? I know you said that you thought you saw her at 3:00 but if not, maybe she did slip out sometime between lunch and 3:00.
Alot of times when cats get out, they look for warm spots and often find shelter in a garage or shed that was open at the time but may be shut now. I run a site for lost pets and always tell people to make sure to check all their own outbuildings as well as going door to door to have neighbors check theirs as well. A man recently sent me an email and pic to post on the site of his lost cat, an indoor girl who slipped out. She was missing for 9 days when the mystery was finally solved. The cat had snuck into the 90 year old man's basement next door when the cleaning lady was doing laundry. She wasn't found until the cleaning had come back to do laundry again 9 days later. The cat survived thankfully and now I also tell people to ask their neighbors to check their basements too!
One other thing I usually suggest is to look for lost cats in the early morning or early evening. It's quieter and provides a better chance for her to hear you and vice versa if she is up a tree or under a porch or something.
And, if you bring a flashlight with you, even during the day, you might be able to pick up the glare of her eyes if she is under a car or in a dark space hiding.
Putting the word 'reward' on your posters is a good idea too. You don't have to specify an amount but it might inspire kids to look for her after school etc.. and the more eyes looking for her the better.
Putting an ad in the newspaper is a good idea too. Usually the first two things people do when they find a cat is contact the local shelter and check the lost and found section of the newspaper. Contacting the shelters is key though because even if they don't have her, they will know to keep an eye out for a cat with her description.
I also suggest to people that if they put some things out on the porch that smell like your cat, blanket or bedding or a litter box, that might help them wander back if they're spooked and a bit disoriented. Also, putting something outside that smells like you might help too. A sweater or blanket, whatever.
I know you mentioned that you shook the treat bag. Maybe firing up the electric can opener might do the trick if she is indeed still inside the house.
Hope some of this is helpful in your search for your Babycat. Here's hoping she's just curled up in a cozy secret spot in the house and comes wandering out, stretching and yawning, wondering what all the fuss is about. Don't give up hope.