When I was in college I was a RA in my dorm. we used to have a real big problem with people pulling the fire alarms at 2 in the morning. We would have to evacuate all of the residents and check every room for fire. It was a huge pain in the rear. After about the 10th or 15th time I told the campus cops that they could put some fluorescent dye on the fire alarms switches that was invisible in normal light, but visible under a blacklight. They put it on and the next time the alarm went off they simply went around and shined the black light on all of the doorknobs in the dorm. When they found the one with the transfered dye, they ask me who lived in there, and when they confronted him about it, he immediately denied doing it, even after the police shined the light on his hands and you could see the dye on them. That was the last night for over a year that the fire alarm was pulled!
Great idea! But it wouldn’t work on a neighborhood scale. And,,,, I like the thought of all the discomfiture the poison ivy will cause!