Don't give up hope. I saw this article linked on another board and it taught me a lot about missing cats. I thought I understood cat behavior but it was eye-opening for me. I quoted it below but left out some parts as it is very long. To read the entire article, go to:
http://www.sonic.net/~pauline/indoor.html
How is a lost indoor-only cat different from other missing pets?
If your lost pet is an indoor-only, very timid, or possibly sick/injured cat, the chances are higher that it is very close by perhaps a mere 1-5 houses away. On the other hand, if your missing pet has been an outdoor-access cat whose normally very regular schedule has been suddenly disrupted, the chances are higher that it has become inadvertently trapped, relocated, stolen, hurt, or ill, and therefore may not be as close by.
If my kitty is close by, why doesn't it come when I call?
When an animal is extremely frightened, its instinct is to hide and not make a sound. Even if nearby, indoor cats or sick/injured cats often do not meow back when their owners go out calling for them. This "complete cover" mode is an instinctive, self-defense mechanism and the cat will do everything possible to stay hidden.
In this state of extreme fear (whether it's the fear of the unfamiliar outdoor world, fear of being chased by a dog, or fear and pain from injury or illness), ANYTHING is perceived as a threat including the sight, smell, and sound of their owner offering a fresh rotisserie chicken. This is one of the hardest things for people to understand how can we, as providers of food and love, be perceived as a threat to our own kitties?
To the frightened kitty, it is not a choice between comfort, food, and love versus fear, thirst, and starvation. It is completely instinctive and automatic. Your cat is not choosing to avoid you, or choosing to starve, and it doesn't mean your cat doesn't love you. In this defensive mode, your kitty believes that in order to survive, everything and anything must be treated as a threat including you. It won't be responding in the same way it responds at home.
Cats who fit this description would be in the Classic Complete Cover personality profile. If you believe your missing cat may be in this category, do not expect your sweet kitty to come running towards you at the familiar sound of a kibble bag. Instead, your missing pet is most likely hunkered down to the ground, more frightened than ever, ready to dart away from any perceived threat even from you.
If my cat is so close by, how come I haven't seen it?
"It's as if my cat disappeared into thin air!" is a phrase nearly every owner expresses.
Universal Rule #1: Cats cannot simply disappear, no more than a house, truck, or horse can simply disappear. Because of their size and instinct to stay hidden, cats can do a pretty darn good imitation of it though.
Universal Rule #2: Just because you don't see your cat DOES NOT mean your cat's not there.
Many owners assume that if they have looked for their indoor-only pet and couldn't see it, the pet must have been taken in, stolen, or must have run to some distant location. While this occasionally happens, it is rare.
When your cat first ran, it looked for the first safe spot it could find. It very well may know where "home" is, but to get from its current "safe spot" (wherever that may be) back to "home" means exposing itself. But this complete cover instinct tells it to STAY PUT and HIDDEN rather than venture AWAY from its new safe spot even if home is only a few yards away, even if the animal is starving.
Universal Rule #3: Just because you don't see your cat DOES NOT mean your cat's not close by.
Many people assume that if their kitty has not been spotted or heard meowing, it must not be close by. NOT SO. Many people have reported that when they finally found their missing cat, their kitty did not make a sound nor move towards the owner.
What are typical behaviors of cats in this category?
These cats are desperate to find a place that they perceive as safe. They will run if spotted or approached, keep very low to the ground, legs short, dart from place to place if they perceive additional threats. They will typically hide during the day and sneak out at night for small bits of food, then quickly dart back to their hiding place. Use this information when you do foot searches and when screening calls of sightings of your cat. Confirm that the markings and location of the sighted cat match your cat. Then ask the caller to describe the behavior of the cat at the time it was seen. If the caller describes a cat lounging in the sun, it doesn't fit the profile of a frightened or timid cat.
What places might my cat be hiding?
The ideal place for a hiding cat is an area with no resident cats or dogs, but with access to food and water. Indoor cats are often found within a five-house radius of their own home. They may move if chased by a dog or a cat, or if they feel threatened, but once they spend some time and deposit their scent in an area, they will most likely stay put. If your cat is a timid outdoor or sick/injured cat, there is a greater chance that your cat will be found somewhere within its territory which can range in size from several acres for a country cat to micro territories for a city cat. If you know the area your cat roams, concentrate your search in this area, then move out from there. Think like your cat. Where would a frightened kitty hide? What areas would be attractive and what areas would be avoided? Think low, not high.
What if I see my cat early on?
In this category of Classic Complete Cover Cats, several things are true:
The cat will likely be closer, rather than farther, particularly early on.
Early on, the cat may show its body, but that is no guarantee it will come to the owner, or allow the owner to approach it.
The longer the amount of time has passed, the greater the chances that something might occur that will scare the animal farther away.
The more experience a cat has outdoors previously, the farther it may venture.
The more timid the personality, the closer it will stay.
Over and over people have recounted that they were able to see their pet the first or second day it was lost, but then the kitty ran off in fear. Many times the owner will not spot their cat again for days or weeks, if they spot their cat at all.
What this means is, as more time passes, either they are less courageous, better at hiding, or are scared farther away where it is more difficult for the owner to spot.
What if I see my cat a month or two later?
Sometime after 4-8 weeks of hiding, some cats begin to show themselves. My guess is that these cats who maintain complete cover so successfully early on also have trouble maintaining their weight. Once their weight drops below a certain point, they begin to throw a little caution to the wind because they are so hungry.
CAUTION IF YOU SEE YOUR CAT: Nothing can be more heartbreaking than to lose your animal, see it, maybe even touch it, but then to have it disappear again. I cannot advise you one way or the other because both behaviors occur frightened cats may run; frightened cats may also stay put. All I can say is please BE FOREWARNED and VERY CAREFUL when approaching your own pet. It may be the best chance you have of retrieving your pet, but it also may be the last time in a long while that you see your kitty.
Some cats allow themselves to be picked up, but many cats scratch and bite in fear, and then run off when the owner is unable to maintain a secure hold. If you decide to approach, make yourself as small and quiet as possible. Wear heavy gloves. Once you decide to pick up your cat, you CANNOT change your mind midstream. You have to commit to the decision, hang on to your cat, and don't let go.
There is no way to predict whether your cat will allow you to pick it up or take off at a run.
