Taking a mushroom from the forest would most probably be considered stealing, indeed. Even though nobody cares about it, that mushroom does belong to someone and does have some value, so after finding it, by taking it home, you stole it.
I don't want to turn this into a mushroom discussion but I was referring to the kind of mushroom that grow by themselves if the conditions are right. Hence, they don't belong to anyone if they are found on public land, not private property. Mushrooms don't have seeds, they have spores - fungal spores. When it's windy, these spores are dispersed to other areas. And if it's humid and wet enough, you could end up with mushrooms in your garden without wanting them. Anyways, enough with the mushrooms. 