As far back as 2012 Polygon reported on the use of Kinect to see spirits when it outlined the show Ghost Adventure's use of the tech. Now there's a far more in-depth look at the practice and some beautifully creepy examples to boot.
As the article points out, the idea appeared almost as soon as the camera did. Sightings have included everything from shadow figures and mist, invisible but detected additional players, and human shaped wire-frames detected by special software. It's worth noting, for the last example, that this story mentions Ghost Adventures' paranormal technology inventor Bill Chappell "outfits the Kinect's 3D motion tracking camera with custom software specifically designed to seek out and read human forms". So a man looking for people shapes writes a program to find people shapes and, unsurprisingly, finds them.
However, head over to YouTube and search for Kinect Ghosts and you'll find plenty of other examples:
The reports also talks to software engineer, security analyst and sceptic Tim Farley who notes that none of this is tested in laboratory conditions. According to him, what isn't a doctored hoax is most likely the sensor picking up false readings from any number of sources and reflections.
Still, it's fun. Apparently you can get the skeleton tracking shown in the videos if you run you Kinect through its Windows SDK on a PC, allowing you to map a wire-frame skeleton to people or (potentially) thin air - which would definitely make an interesting attraction at your Halloween party.
You should definitely check out the whole thing, as it's a lovely, creepy and interesting read.