I lost my Shu a few months back (teeny tiny female childreni). She turned up happy and healthy two nights ago! I've heard similar stories about months passing, hope being lost then the snake suddenly turning up again from others aswell. I hope you find her asap, however don't give up hope.
There are some fantastic suggestions here. All I can add is to look and look in the smallest spots. I have two yearling intergrades who get out on a fairly regular basis and they always tend to go vertically, rather than horizontally. So perhaps check up on top of tall furniture and stuff like that. Also, some like to jam themselves in between the wall and the back of a flat piece of furniture like a book case. They can travel up and down this way.
If you suddenly notice the birds outside congregating in the one spot and kicking up a real stick, check it out quickly!
Goodluck, Garlo. I really feel for you. It's so distressing to lose them, isn't it.