In Lepage and Brassard's Polygraph, the Russian Matruska (also known as a matryoshka) is a "nesting" or "nested" doll. A large wooden doll opens up to reveal several layers of dolls within, each one smaller than the previous, each fitting inside the next larger doll. David gives this doll to Lucie to celebrate her acting job.
However, what makes the doll interesting—and a viable piece of the developing plot—is David's reaction to the doll. For him there is more than a souvenir on the table before them. In fact, his ideas are unsettling: they speak not only to the secret David keeps (which he asks Lucie to keep from Francois), but also to the details of Francois' own heartache, his ultimate suicide, as well to the unpleasant idea that feelings and truths can be hidden beneath many layers. In David's line of work as a criminologist, as he collects information regarding death, it brings to mind the distasteful idea that anyone could be hiding any kind or number of secrets, all buried far from sight.
...I like to think it may stand for other things like...Hidden feelings...One truth which is hiding another truth and another one and another one.
Because of Francois' unusual behavior, specifically his "violent and lamenting cries from the other side of the wall" in Lucie's apartment, one is left to wonder if Francois is hiding something. One cannot help but consider whether he killed the girl or not. We know he found the body. He admits he was a suspect. David reveals that he gave Francois' polygraph test. And while David believes the polygraph cannot be tricked, we know that these tests results can be physiologically altered. Francois even admits to Lucie that he can't be sure anymore if he killed the girl or not. This haunts him and may be what drives him to step in front of the moving train.
However, David purposely does not tell Francois that he is no longer a suspect. Cruelly, it is something the authorities do on a regular basis. "It's a strategy to keep everyone in ignorance." They never tell those they test that they are no longer persons of interest in a case. And so the concept of "hidden truths" applies also to David. The audience is not so sure it can be sympathetic to such a man who says he and his colleagues are not "violent brutes," when in fact, they are.
And just for fun, we should not forget what might be foreshadowing—when David notes that in many "who-done-its" the killer is actually a policeman! Recall that David tells Lucie:
The men leading the field of criminal research are very, very intelligent; a fact you will never see in a thriller. It's too frightening perhaps.
One wonders why all of these details are included—unless they simply support the idea that anyone can keep secrets deeply hidden.