I'm familiar with this custom in Chabad shuls to recite this mishna (Mikvaos 7:7) at the end of davening (after Aleinu, mourners' kaddish, and "al tirah") in memory of the deceased:
After reciting that mishnah, the passage "Rabbi Chananiya" is recited, followed by Kadish DeRabanan:
In my experience it's all read super fast and slurred together, so it's hard to catch what's being said.
You mentioned that this is before mourners' kaddish at the beginning of davening. I'm not familiar with any nusach that has a mourners' kaddish at that point; however, many synagogues have a short bit of Torah learning between mincha and maariv when they're recited together. After this, the passage Rabbi Chananiya from above would be said followed by kaddish DeRabanan.