The annual re-inspection came from the days where deterrent pesticides (aldrin, chlordane, dursban) were used and all are banned now in most places. It was a task of labor indeed. Drilling slabs along walls, around every slab penetration in the basement, wall voids, behind brick veneer, rodding soil, etc. Most wouldn't provide the guarantee if the structure had direct wood contact with ground either. Those pesticides, while effective and long lived, repelled the insects. If there was a gap somewhere in the treatment and there always was with challenging construction types, the buggers WOULD find it and make their way in and start there quest for foraging on timber fibers. This is why to have a guarantee plan there was an annual re-inspection with a fee. As long as you paid this fee and inspection, you were covered for both damage and follow up treatments.
With fipronil based termiticide, the insects don't detect its presence and it takes time to kill. Termites being social in nature will quickly spread it around and the colony will be eliminated in a few months. Incomplete/partial treatments can be effective in this manner. The residual termiticide remains active for up to ten years. You may or may not need to re-treat depending on construction and how termites found their food in the first place. Most construction work is the source - they like to bury wood scraps under dirt filled porches or in space where slabs get poured. This wood brings them within a few feet of sill plates, for example. Storing firewood on the ground next to a building is another no-no.
The warranty and re-inspection isn't really a requirement with fipronil treatments. If you see swarmers the following spring AFTER such a treatment it wasn't effective and I'd question the pest control operator's abilities at that point.