Complex sentence: Emma finds no signs of a break-in, but hears rattling coming from the heating vent, where upon opening it, she finds a hunter's knife inside, which matches the weapon that was used in the murder.
Split into simpler sentences: Emma finds no signs of a break-in, but hears rattling coming from the heating vent. Upon opening it, she finds a hunter's knife inside, which matches the weapon that was used in the murder.