In Foucault's Pendulum, semiotics plays an integral role in the construction of the narrative, as it does in all of Umberto Eco's novels. The two book editors employ a computer program into which they feed various pieces of information, and the program produces a narrative based on that information.
The editors, Belbo and Diotallevi, feed mountains of information from various often esoteric sources into a computer program, and the program sifts through the information, constructing a coherent narrative. This activity forms the very basis of the plot. In sifting through the information, the computer program is employing semiosis, the interpretation of signs. The program must determine not only what a given piece of information means but also how it connects with the other pieces of information. Taking the information to mean a collection of signifiers, the computer determines what those signifiers signify.
One example of this activity involves the Knights Templar. The editors input various pieces of information, from the rounding up and torture/execution of the Templars in the early fourteenth century, to the role of the Templars in the Holy Land during the Crusades, to the supposed location of the legendary Templar treasure. Inputting other forms of information concerning the Holy Grail - its nature, its potential resting place, et al. - allows the program to synthesize this information and essentially conflate the two narratives into a coherent whole. It can only do so by considering the meanings of the various pieces of information and any consistencies in the two narratives.
Belbo and Diotallevi also find themselves inputting information from the Kabbalah and secret societies, and the program conducts the same interpretive activity, decoding the various symbols and stories and adding it to the growing collection of information.
With Foucault's Pendulum, it is not really possible to point out very specific examples of semiotics, because the very nature of what the characters are doing throughout the novel stems from semiotics and semiosis (the interpretation of signs). The resulting narrative is not distinct from semiotics.