Have you heard of passports, which are generally marked when people enter or leave countries?
My experience was that when I entered France from Africa, my passport was stamped for France but not for the country I left. My experience was also that many countries stamped passports with an Arabic script that I would not expect customs officers to be able to read, and that the stamps do not go in order. I don’t know if that is still true, but it used to be true. It’s quite possible that our customs officers would not figure it out if a traveler lied about travel to Liberia, even one with a Liberian passport.
“Oh, I’ve been studying in Paris all year, and they stamped my passport when I came back to France from Italy.”