Termite barriers protect a building by forcing termites to show themselves, as they will often build mud tunnels around termite barriers to reach the timber above.
The presence of termite tracks or leads does not necessarily mean that termites have entered the timber though. A clear view of walls and piers and easy access to the sub-floor means that detection should be relatively easy.
However, many styles of construction do not lend themselves to ready detection of termites. The design of some properties is such that they make the detection by a pest inspector difficult, if not impossible.