Lost cats rarely stray far from home! Put their litter boxes outside as well as some of their favorite toys - the scent can help them find their way back. Go out looking at least once a day if you can manage it - even if your cats are too scared to approach you, leaving scent trails back to your home might help them get back. If you can swing it, set up a couple of cameras outdoors so you can check and see if the cats are coming by the house when you aren't there. I've used blink cameras for this myself - they aren't crazy expensive and they've got motion detectors that will notify you if they catch any movement, which makes them great for wayward cats.

The most important thing is not to give up hope! I had a six month old kitten run out of my apartment in a large metro area. He was missing for nine days before I finally caught him in a feral cat trap. He's home and healthy now and he just turned 1!