ᐅ Room smells like marijuana or hashish

Good evening,
We have the following problem: sometimes there is a smell of marijuana or hashish in the children's room. We recently bought the house. It is a prefabricated house with a solid basement. The children's room was completely renovated, meaning new flooring, new wallpaper, and a dropped ceiling installed. This smell does not occur all the time. We cut some holes in the wall to check whether the mineral wool insulation was okay, which it is. The insulation is dry and does not smell like that. We also found no animals or anything else. Please help, as we are getting desperate. Oh, and no one in our household or the neighborhood smokes marijuana.