In a normally busy school at night, why can silence itself feel like a warning?

Answer: Familiar voices disappear, leaving you unsure whether people are nearby.

Silence muffles approaching footsteps, making other people harder to hear. — Silence does not muffle footsteps; with less background noise, a footstep may be easier to hear. The uneasy feeling can begin before any footsteps occur. What is missing is the familiar sound of people, not the ability of approaching sounds to reach you.

Darkness makes the ears physically more sensitive to sound. — Darkness does not explain why missing familiar sounds can itself feel unsettling. You may pay more attention to hearing when you cannot see well, but that is different from your ears becoming physically more sensitive. The relevant change here is what the silence suggests about whether people are nearby.

Familiar voices disappear, leaving you unsure whether people are nearby. ✓ — Missing familiar voices can make the presence of other people uncertain. In a school, conversation and routine activity normally tell you the place is in use and someone may be nearby. When those expected sounds disappear, the uncertainty can feel unsettling; the same quiet is less surprising in a place where silence is expected.