A competent inspector should know enough to understand that there is almost always a small possibility that an inspection could, no matter how thorough or competently done, miss some termites.

There are several reasons for this:

• Termites start a new colony as a couple, they dig their way into wood an often leave no visible sign of their presence for more than a year.  Two termites hiding in a crack in the wood can easily be missed.
• Termites like to work deep within timber.  They can be present for weeks, months or even years before creating enough damage or excavation so as to be easily detected.
• There are nearly always places in a building where full inspection was not possible.

A good inspector will not miss anything serious.  They'll list all the suspicious bits and the places where they couldn't check.  Especially for a pre-purchase inspection.  You can be confident that it is most unlikely that any major infestation is present but not reported, though you can almost never be 100% sure that there are actually no termites present.  That's why they won't write that there are no termites in the house.

It's another good reason why you should have regular inspections.  That way nothing major should occur without you knowing.