I remember a while back seeing a date line that revealed some really disgusting stains left behind in hotel rooms. They used a special light to expose them.
I had some invisible ink/UV florescent pens some years back in high school. Multiple florescent blacklights (it was peaceful) and many talented friends. You want to see a neat freak have a conniption fit, subject them (permit them ) to a self requested blackout w/UV supplementation and a mirror.