I have similar experience before and asked several termite companies to tackle it but the termite came back every year.
At last, an old friend of mine introduced his friend from mainland china. The man only a visitor to hk and went back to china now. However, (i dont know where his experience came fro), he managed to tackle the termite problem in my house and it never comes back at least for the last 4-5 years:
His wisdom and method perhaps I can share with you to see whether it help:
(a) first, any termite problem and the serious degree of the problem will depend on whether the "Queen" is in the house or elsewhere. If not, then the degree of spread can be tackle more easily. The only thing is to check where the route to the house is. If the "Queen' is in the house, then you really need to find their host.
(b) he only use "cyanide' and mixed it with sugar. The mixture was put at various locations along the walls. However the mixture is posionous and he placed it on a very small paper dish so as to confine its position.
(c) after a week or so, i cannot find any trace of termite in my house whatsoever.