oh_riginal said:

I always assumed that the Star Destroyer couldn't detect the location of the Falcon simply because it was in a sweet spot where it could be parked ON the much larger ship, making it appear that the Falcon is just another piece of a huge mass on their radars or sensors, or whatever they were using. Not to mention they were in a nice spot where no naked eyes inside the ship could see them hiding there.

I feel it doesn't need to be any further complicated than that. After all, we've all understood that scene without confusion since 1980, or whenever we first saw the movie.

The Falcon has a cloaking device.