Homenum Revelio

Homenum Revelio is one of the most useful detection charms in the standard curriculum, and one the trio leans on heavily during the year on the run. Cast as a slow wand-sweep across a space, it produces a sensation in the caster — most often described as a faint pressure or a quiet hum — when other humans are present. The charm doesn't say who. It doesn't say where exactly. It only says that someone is here, somewhere.

What makes the charm so useful is precisely that it does nothing else. There is no flash, no visible effect, no announcement to the people being detected. The Death Eaters and the snatchers Hermione casts Homenum Revelio against during the Horcrux hunt have no idea they were just scanned for, even when they're standing in the next room. The charm is a one-way mirror, which is exactly what you want when you're trying to figure out whether the safe house you're considering still has people inside.

The charm's most memorable in-book moment comes early in Deathly Hallows, when Hermione casts Homenum Revelio inside Number Twelve, Grimmauld Place after the trio realizes they've been followed. The pressure she feels — confirming someone else is in the house — sets off a tense sequence that ends with the discovery of Mundungus Fletcher's intrusion and ultimately leads to the trio realizing where the real Slytherin's locket has gone.

Homenum Revelio has limits. It does not detect non-human magical creatures, ghosts, portraits, or magical illusions. Strong concealment magic — the Fidelius Charm in particular — can hide people from it entirely. And the cast has no precise direction or distance reading; the caster knows someone is there, not where exactly to find them. But for the binary question of "are we alone?" — which is often the only question that matters — the charm is essentially purpose-built.