You reminded me of this story I read a while back about how an indoor pot farm was identified because the heat from the grow lights melted the snow on the roof and apparently in the Netherlands that was enough to get a warrant - https://www.telegraph.co.uk/news/worldnews/europe/netherland...

While looking for that story, I came across a wiki article about a similar story in the US, although the police there used thermal imaging to identify the farm from outside the home. That was ruled that constitutes a search w/o a warrant in the US. https://en.wikipedia.org/wiki/Kyllo_v._United_States I would suspect observing suspiciously melted roof snow in the US wouldn't violate the law like using thermal imaging would.

I don't know German law, but I could see how that could get you a search warrant if Dutch and American law are any indication.