The rabbi was intent on conveying one message in his weekly shiur. If you use a sefer from the shul library, put it back in its place before you leave the synagogue. He said that not putting it back in its place was a lack of respect for the sefer itself, as well as for the congregants who might be searching for the sefer. To reinforce his point, the rabbi related an anecdote about the Steipler Rav who was walking home from shul acccompanied by another man. Midway to his house, the Steipler told the man that he had to return to the shul. The man accompanied him back and witnessed the Stiepler returning to his shtender, picking up a sefer and returning it to the bookshelf.
A second incident the rabbi attributed to the Steipler was when a pregnant woman approached the Steipler for a blessing, hoping for a normal delivery since all her other children had been born by cesarean births. The Steipler counseled her to return home and check if any of the sefarim had been turned upside down in the bookshelves. She and her husband found three sefarim that had been turned upside down and they carefully placed the sefarim in the shelves right side up. A short while later, the woman gave birth through a normal delivery.
Let's try to be considerate of others. By not putting a sefer back where it belongs, one is showing lack of respect, as well as causing bitul Torah to the person who, instead of spending his precious moments learning Torah, must waste time by searching all over the shul for the sefer.
Needless to say, we all carried our Chumashim and Siddurin to the bookshelves and placed them back where they belonged.