They say that you never forget the smell of human remains. Dead human bodies have a specific, unique scent, unlike the rotting meat of any other animal. Those familiar with it can extinguish and identify it without much difficulty.
Yu Qingyu, unwillingly, was very familiar with it. When he was living in the zombie apocalypse, this scent surrounded him each time he left the shelter. The experience was so deeply engraved in his mind, that it might follow him for a few lifetimes.
That's why, when the stale air filled with a nauseating scent carrying a note of sickening sweetness hit his nose, he immediately knew. It was still fairly faint, clearly coming from a different room, further away, but he still could tell. There were human remains nearby.
He snatched a few suppressing talismans from his pocket, just in case, ready to defend himself, and started looking around the room.