If this is a common occurrence, the best thing you can do is wait it out. If your cat still hasn't come back after a week, you can start putting out flyers. Since it's only been a couple of days, it's a bit early to be jumping to conclusions.
Has your cat been fixed? If not, it's possible (but unlikely) that she got knocked up. Pregnant cats often look for dark, secluded places to give birth. If your house doesn't have enough dark, secluded places, she may have set off to find one.
It's a longshot, I know, but there is the slight possibility if your cat has not been properly spayed.
Put up posters, sit back and wait. If, as Ernie said she's pregnant, she'll stay away for a while. She might have got a scare and hid somewhere and will come back when she's relaxed. Or maybe she has a lover ;o
My friend lost his cat once. A week after he went missing, they were walking down the street and heard him meowing. Apparently he fell in a hole in an abandoned house on the street and couldn't get out. They called the police, they broke in and got the cat. Then the house was then found out to be a biohazard due to some dangerous fungi growing in it.
