Well - if she didn't nest near your home, I'm not sure what to suggest. It's important that you not go looking for her, however, as if you find her nest, it'll make her feel insecure and she'll have to move the kitties to a new "home."
The only thing you can really do is start putting food out near where you think she may be. This, of course, will attract other cats or wildlife, so if there are either many pets whose owners let them be inside/outside cats, or if there are lots of strays or ferals, this will attract them, too. But if you want to start trapping, putting out food to attract the cats is the place to start.
If there are other cats, and you do decide to begin trapping to have them spayed or neutered, please make sure you have the vet "ear tip" the cats (unless you know they're owned). Even though you ought to keep a log of the cats trapped and fixed, this way you'll know in an instant if you've retrapped a cat you've already had fixed.
However - since you haven't seen her for a while, at this point it's probably best just to provide food for her and the kittens, and wait until the kitties are about 10 weeks old to trap them and mum. If you're not sure you'll be able to have the kittens adopted out, and you'll have to return them after being fixed (which can be done safely as early as seven weeks old, though many vets won't do it when they're that small), then they really must be with mum for 12 weeks before you trap them and mum. It's at about 12 weeks that they're done learning what mum's going to teach them.
Feel free to post with any other questions.