While sorting out some old odds-and-ends of my children, my wife found a little electronic motion detector gizmo. If you turn the device on, and move in front of the beam sent out by the device, the device will give off an alarm.
I decided to use this device in a kind of primitive "spirit presence" test. I turned on the device, and placed it on a window sill. I pointed the device towards a wall about one meter away from the device. Given such a position, the only way a human could set off the alarm would be if they placed their body or part of it in the small space between the window sill and the wall.
The alarm went off several times in the hour after I started this test, even though nothing visible moved in the small space between the window sill and the wall, or within two meters of such a space. I tried moving within a meter of the device, and this did not set the device off. I could only set off the alarm by placing my hand in the small space between the window sill and the wall. There were no flying insects in the house. The alarm did not go off in the second hour. Sudden changes of lighting (such as turning on a light in a dark room or using a camera flash near the detector) can cause such a detector to go off, but there was nothing like that when the alarm went off.
Postscript: In early November, I quite a few times heard this motion detector sounding its alarm unexpectedly, when there was no motion anywhere near it. Twice this occurred when I was in the bathroom, with two walls and twenty feet between myself and the detector, and no one else in that space. The second time this happened I checked the radiator, to see whether it could conceivably be heat from the radiator causing the motion alarm to go off. But the radiator was not warm.