A roughly three-to-four-foot garage-floor area that faintly reacted to luminol but did not react to phenolphthalein when sampled

Ertl described a roughly three-to-four-foot garage-floor area that glowed faintly under luminol but had no visible spots or stains with the lights on. Swabs from several reacting places did not react to phenolphthalein, and he could not identify the substance responsible for the luminol reaction.