Interference with the enjoyment of property is a legal concept, which like many other legal concepts does not mean what a lay person might think it means. The fact that a landowner is unaware of an intrusion does not mean that the intrusion can't be an interference with enjoyment of the land (in other words, a trespass). Suppose, for instance, that you tiptoe across someone's back yard late at night, or while the owner is on vacation. Is that a trespass? Of course. Does the owner's lack of awareness of the activity mean it isn't a trespass? No. This would be equally true if you drilled a well at an angle so that it passed under someone else's land. That's clearly a trespass, even if the owner was not aware of it.

To sum up a whole body of law in a reasonably accurate way, you trespass on someone's real property if you cross (or cause something to cross) an invisible line (which lawyers call "the close") around the border of the property. It doesn't mater how high or how far down you do this, subject to the exception for ordinary aviation activities. It was plain that that exception had to be created, lest landowners be able to sue airlines for trespass every time a plane flew over. The question of how much that kind of activity interferes with an owner's use of the property arose in connection with determining the scope of that exception. It was clear that flying 10,000 feet overhead shouldn't be a trespass and flying five feet up should be, so the issue was where to draw the line in creating that exception. Causby did not, however, rewrite the whole common law of trespass to require that trespasses have to bother the owner, or even be something the owner was aware of. Indeed, it couldn't have done that, because the law of trespass is state law, and the Supreme Court doesn't get to make (or remake) state laws. It is therefore absurd to argue that activities aren't trespasses just because the owner of the property is unaware of them. Now, it will eventually turn out that unmanned aircraft will become so routine that an exception for their crossing someone's borders will have to be created. I don't know how broad or how narrow that exception will be, though I very much doubt that it will be broad enough to include ordinary model airplanes. Nobody else knows either, and anyone insisting that he does is a fool.