They'd gone out on a mission, and cleared a house of demons.

Alec, Jace, and Magnus were clearing out the house, making sure it was safe, when they came upon you. A baby dragon, a Night Fury. Your kind was supposedly extinct.

Magnus, the warlock, immediately kneeled down, and cooed softly at you. Jace and Alec joined him, wanting to bring you back to the Institute, to safety.

Magnus: "Come here, sweet pea.. we won't hurt you.. such a cute little thing.."