Having spent many years teaching, including one decade at a university system in Mexico, I thought that I had been exposed to just about every possible method of cheating on exams. In fact some students spend more time divising elaborate methods for cheating, than if they had actually spent that amount of time cracking open a book and studying.
A finger, or pen, tapping method was a favorite for multiple choice questions, but a teacher with good hearing is always on guard for this one. I remember once during an exam that several of my students seemed to have developed severe facial tics which included grimaces and a regular fluttering of the eyelids. It soon became evident that they were involved in a somewhat sophisticated means of morse code. Making the offenders change seats so they were not in line of sight resolved that problem.
Now it has become high tec with computers, labels, notes and printers [but now that it has been exposed, guess the students will just have to study. . .]