The Ticking Pulse Murder

Data extracted from the victim's health-tracking ring. It shows a heart rate fluctuating between 72-85 beats per minute until 13:55. From 13:55 to 14:45, the pulse stabilized at exactly 60 beats per minute (without any decimal fraction or variation). At 14:45, the pulse suddenly dropped to zero.

A classic spring-wound mechanical metronome found on the victim's desk inside the vault. Its metal arm has completely stopped moving, and the sliding weight on the arm is firmly locked at the (60 beats per minute) mark.

A recording from the vault's intercom system at 14:15. The external voice asks: 'Is everything ready for lockdown?'. The voice from inside the vault replies: 'Yes, do not interrupt me'. Spectral analysis of the internal voice is a 100% match with an old audio file named 'meeting_reply.wav' found on a USB flash drive left plugged into the main console outside the vault.

The window in the east warehouse is wide open. The floor and goods directly beneath the window are completely clean and free of a single grain of the thick red sand that covered every inch of the building's exterior facade.

A mug in the break room. Its bottom contains a thin layer of dry, cracked coffee. The mug is completely cold at room temperature and entirely dry, confirming that no hot liquid has been placed in it for at least 3 hours.