When the soldiers returned from the war with Midyan, the Torah tells us that Moshe Rabbeinu, Elazar HaKohein, and the princes of the shevatim all went out to greet them. Rashi explains that some youngsters had run to grab some of the abundant spoils. Therefore, Moshe Rabbeinu and the other leaders personally went to prevent this from happening. When it came to a suspicion that someone was stealing, Moshe did not rely on sending a messenger to investigate. Rather, he felt it necessary to personally stop such behavior.
Rav Wolbe (Shiurei Chumash) notes that throughout the Chumash we find the great vigilance that our forefathers displayed when it came to matters that involved other people's property. Avrahom's animals traveled with a muzzle lest they graze in someone else's field. Yaakov lived in Lavan's house for more than twenty years and he didn't take a single item from the house. Additionally, we find that when Moshe went to collect the money for the building of the Mishkan, he wore a specially tailored garment that had no pockets or seams, lest anyone suspect that he might take some of the money for himself.
Rav Wolbe related that one Yom Kippur before Ne'ilah the Chofetz Chaim stood in front of the Aron Kodesh and gave a shmuess. He said that twice in Ne'ilah we state, "So that we may refrain from the oshek of our hands (money taken unlawfully)." We see the seriousness of stealing. Since Ne'ilah is a time to make a kabbalah (resolution), we should resolve that if we should have any money that belongs to someone else we will return it immediately after Yom Kippur.
Stealing is a terrible aveirah and yet we don't even have any idea what is included in this prohibition. For this reason, Rav Yisroel Salanter urged people to study the third section of Choshen Mishpat which deals with interpersonal monetary matters. Rav Wolbe heeded this call and instituted that the Kollel that learned in his Bais Hamussar should study those specific halachos. If we want to be sure that we don't unknowingly transgress the laws of gezel, we should take at least a few minutes a week, to study these halachos and gain a more comprehensive understanding of them. (Recently a number of excellent books have been published in English on these topics.)