If anyone is interested, I just purchased several green and blue field lasers. These are the ones you've seen on Ghost Hunters and Ghost Adventures. They project a laser field across the walls and floors so that you can tell if something passes through the field. I purchased green and blue so that you can combine two from different angles to get a 3D representation of the location of the disturbance and also use multiple lasers sourced from different locations without interfering with one another.
I've got a handle on more of them if others want some. I intend to keep at least one of each.
