Police in Holland know when they see a marijuana “grow house” in the winter from the outside. Grow houses obviously generate heat and when there has been a recent snowstorm, melt the snow off the roof. The police also had a good reference point since it was an apartment complex where snow remained over the other units.
In this case a major grow house was discovered within the apartment complex leading to arrests. Holland is very tolerant of marijuana use but remains vigilant in eliminating non-regulated grow-houses and anyone otherwise not in compliant with Dutch drug laws.