Muggles' Guide to Harry Potter/Magic/Homenum Revelio

Overview
Homenum Revelio is a spell which will reveal to the caster whether any other (presumably living) humans are within its area of influence.

Extended Description
When they first arrive at Number 12, Grimmauld Place after leaving Bill and Fleur's wedding, Harry, Ron, and Hermione are concerned that there may be Death Eaters lurking. Hermione casts Homenum Revelio to determine that the house is, in fact, empty.

When Harry, Ron, and Hermione are betrayed by Xeno Lovegood, the two Death Eaters, Selwyn and Travers at first do not believe Xeno's protestations that he actually has Harry Potter. Selwyn determines that there is, in fact, someone else there by using this charm.

In both cases, the person casting the spell reports to the others there present what has been discovered, and Harry, Ron, and Hermione are only aware of the spell because they hear it being cast.

Analysis
The fact that Hermione reported nobody present at Grimmauld Place, and that Selwyn had to tell Travers that there was someone "up there," leads us to believe that the response to the spell is local to only the caster; other than the person casting the spell, nobody is aware of the results of the spell.