This list had some recent discussions about finding cameras using retroreflective techniques.
For those who haven't used the Spy Finder ($3K from David Cremer, SEA, (505) 546-2833) it is really interesting. As one who has spent some time watching folks design stuff like this I can say that SEA has taken care of things that the rest of us hadn't even thought of. I find it a very useful tool.
The device uses 2 630 nanometer diode lasers. One is on the line of sight and has a low duty cycle. It causes a blink to be seen from a retroreflector or properly aligned mirror. The 2nd laser is off the optical axis a bit and does not. The 2nd laser is used as a fill; it is on when the 1st one is off. This means that there is roughly uniform illumination on the target. It's a clever idea because it removes the distracting blinking of common things like doorknobs.
This retroreflector stuff works because imagers (with film, CCDs, CIDs, retinas,...) cause incident light to be scattered from the image plane. All of that scattered light that strikes the lens exit aperture is directed back to the source (your eye behind the Spy Finder). It's easy to see how this works but more difficult to build an instrument that really does the work.
My own attempts to make this sort of device yielded OK detection of overt TV cameras -- but didn't see the common pinhole variety. The Spy Finder works acceptably even on small lens, wide field-of-view cameras. This camera configuration typically has an entrance aperture that is less than 1 mm in diameter.
An interesting aspect of our work is the difficulty of letting clients appreciate the technical work that we do. Even though they don't usually understand the physics of the Spy Finder, most clients instantly appreciate it after a demo. It helps them feel good because they can actually see what is being done. It's not quite as valuable a marketing tool as a portable x-ray machine, but close.Gordon Mitchell Future Focus, Inc Woodinville, WA (888) BUG-KILR Electronic eavesdropping detection & finding clues in computers.