Just noticed you are here in the OC. Ground termites are very common here. Depending on how lucky you feel you really are, ground bait traps and marginally successful. Also keeping things like trees and shrubs off the house is also a good idea. Trees and shrubs act as a bridge into your house.
I have been there done that with moderate success. With termites in certain parts of the country, it's not if, it's when... With the weather changing from cold and wet to hot like it is now, I suspect many people are about to find out they have termites. A weather change like we are having now often triggers a swarm...