When the Enterprise finds one guy who may be two guys (or two guys who may be one guy) on an uninhabited planet along with an undetectable source of radiation, it’s up to Kirk, Spock and McCoy to figure out the intricacies of “The Alternative Factor.”
Dude, the sensors have a range. But if it’s in range, and not in an alternate universe, it can sense it. And, like, there’s totally stuff that can block them. I’m just saying, if there’s a thing, and we can see it, but the sensors say it’s not there, we can pretty much guess where it is, and in the universe isn’t it.