I Sense Something...
When the Falcon is captured by the Death Star, Vader says he sensed a presence. Of course, we know that it is Kenobi he senses. But if we take all the information we have gained in TPM, then he also senses his son, his daughter, the droid he made as a child, and the R2 unit he first flew with. Kind of odd don't you think?
Re: I Since Something...
Vader also senses Luke in the fight scene over Death Star, "The force is strong with this one." Kind of odd he dosent sense Leia though, but maby they cant sense things if they use the force. Like Luke had help from the force, and used it in the fight at Death Star which made Vader able to sense him. Anakin might use the force,unawere of it, when he asked Qui-Gon all the questions. Leia might not take use of the force because of her strong personality and share willpower. She has been trained for leadership since a very young age.
Sensing can only involve "The Force"...
I think that only users of the Force are able to sense other users of the Force. Kind of like an intergalactic conscious if you will. Makes plenty of sense to me. Simple, here is the layout:
Leia was unaware of her having the force, once Luke says she has it she says "In a strange way, I knew", but she never actually "used" the force, therefore she was unable to be sensed by others. Vader didn't ever sense Luke's existence until after Beru and Owen were killed and OB1 took Luke as an apprentice, therefore Vader sensed Luke only then! I think all other aspects of familiarization (C3PO, R2-D2) were not sensed, just merely coincidental if he recognized them, since being robots, they didn't have any actual thinking or conscious of their own.