I remember years back when someone was trying to stick a claim in against me for causing dampness in their property after we spanned across their roof with quite a few tons of scaffolding, but I knew we hadn't even touched their roof as the beams used were hoisted into position. My insurance came down, and first noted the near freezing temperatures in the building as it was a holiday let and mostly empty in winter and said that if it wipes off, it is likely environmental whereas if it doesn't, its likely soaked through from an issue elsewhere. In my case, it wiped clean and I never heard another word about it.