My guys have been outside for a few weeks now, and lo and behold my biggest male Sambava's cage was wide open went I was sprinting out the door to work on Monday. CURSES!!!!! I don't know what happened, but the door somehow got open. It was definitely shut and latched when I came home the barn when I sprayed the cages, but an hour later when I came out for work after showering etc inside, the cage door had swung open. I can only guess the spray of the water somehow undid the cage latch.
No luck so far. Mine is a big GREEN Sambava, and we have GIANT trees in our yard, with his cage along a ~100 ft row of privets. Needless to say, no luck so far 
It is summer, and chameleons can easily live and even flourish in this weather. There are many stories of MIA chams turning up months later, so don't ever give up! Mine is so big and nasty I don't have to worry about the nieghborhood cats or any human sneaking up on him....he's a really aggressive chameleon!
Some advice: (1) call your local vets and put out a "lost chameleon" bulletin. People may spot the cham and not knowing who else to call, may call the closest vet. (2) take a spare mesh or wire cage and put in some big crix and put it near where your cham was last seen. With such great vision the cham may be able to spot "cricket movement" form a distance and be enticed over to something he recognizes as easy food.
Good luck, and don't give up! Tell all your neighbors. My favorite, albeit obvious, question from an innocent neighbor has been: "so what color exactly is your lizard?" (Sigh...)
~Kerry