The Trace of Emptiness

The victim is lying in the middle of the room. The wooden floor is completely covered in a smooth, flawless layer of resin with no footprints. However, when the forensics team lifted the body, they found that the rectangular patch of wood directly underneath the body was dull and completely devoid of a single drop of resin. Furthermore, the victim's back and the back of his clothes were completely clean of any resin.

The type of resin used is slow-drying, requiring exactly 4 hours to dry and become walkable. Any pressure applied before this time leaves permanent footprints and indentations. An empty resin bucket and a large paintbrush were found carefully washed and left in the hallway restroom sink. A heavy warning sign ('Wet Floor - Do Not Enter') was hung on the outside door handle. The door features a spring lock that locks automatically when pulled shut.

The only camera in the building covers just the main exit door to the street. The footage shows a person wearing a hat with their face covered, running suspiciously while carrying a small box at 13:05.