In ESB, at the end of Hoth's attack, Vader saw the Falcon take off.
He's looking for Luke Skywalker. For him, Luke is on the ship, according to this:
"Our pursuit ships have sighted the Millennium Falcon, my lord. It has entered an asteroid field.” “Asteroids don’t concern me, Admiral,” Vader said as he slowly clenched his fist. “I want that ship, not excuses. How long until you will have Skywalker and the others in the Millennium Falcon?”
(ESB - Novelisation)
Why is he certain that Luke is onboard of the Falcon? He hasn't seen him...
Has he sensed, via the Force, that a Skywalker was onboard?
If so, was it not Luke, but rather Leia?