Okay, clear your mind...
We're about 40 minutes or so into the Empire Strikes Back. Han Solo has just found the safe haven of a deep hole in an asteroid, and a few frames later Luke Skywalker enters Dagobah orbit and scans the planet.
"No technology. Massive lifeform readings though. There's something alive down there."

So, how come Skywalker, using only his X-Wings sensors, could detect lifeform readings on a planet from orbit, and yet Solo and the gang couldn't detect a space slug the size of a Rebel Transport while flying right down its throat?

Answers on a postcard to the usual address...