I was thinking of a poison oak/ivy mash for the sign.
Invisible and the perp would be obvious in a few days unless they’re like me and immune. But the immunity can go away any time. DH used to get it head to toe for weeks any time he touched but after months of drinking goat milk from goats who were fed poison oak he got immune. Now more than 15 years after the goat milk cure he may get a few spots of itch from tools, boots etc that got poison oak on them but it doesn’t spread.
Slide on the poison ivy/oak/sumac
A few years ago I found the homeopathic toxicodendron (poison ivy) pellets for arthritis. I had been told I needed double knee replacements and 1 hip replacement due to arthritis. Haven’t had joint replacements yet AND I am now immune to poison ivy. I’ve read that roughly 35-50% of people who take the homeopathic toxicodendron develop urushiol immunity. I count myself very blessed indeed as I now walk in the woods much easier.