This is a very clever advertising campaign for 13th Street Suspense Channel. A bar in Hamburg, Germany installed a bloody crime scene in the bathroom that can only be seen with a special black light. When a person enters the bathroom, the regular lights are turned off and replaced by the black light.
All of a sudden the messy crime scene can be seen all over the floor and walls and the message "See what others don't see. 13TH STREET. The Action and Suspense Channel" becomes visible. Very clever promotion.