One thing I've always wondered is how Vader didn't sense either Luke (or Leia's for that matter) force presence on the first death star. I guess he was super caught up in with the whole "I sense the old coot who cut off my limbs amd left me to burn" thing, but I like to think obi-wan was masking the pair by just being obnoxiously loud with the force (the force equivalent of him walking into a room with multiple air horns and a boom box blasting away)
Wow, I had THOUGHTS about this one.
Honestly? I'm not a fan of the assumption in a lot of fic and by fans that it's super easy to passively sense someone in the Force. I have used it myself, of course, but I say from experience it restricts writing a lot while... not actually having a ton of backing in the films? (I'm referring to the films here, not to any other media, just because when I want to analyse lore, I only take them as canon; when I'm just looking for cool worldbuilding for a fic, though, anything goes.)
It's not just Leia that Vader doesn't sense on the Death Star. He doesn't sense Luke either, even though Luke is right there screaming when Vader kills Obi-Wan. It's more of an issue with Leia ofc because he literally used the mind probe on her but only finds that "her resistance... is considerable". An easy answer is that I headcanon Leia is a naturally good shielder, which is why she was never found on the Death Star and also why all of the Jedi in ROTS were surprised there were two babies - Leia had been naturally shielding herself, so they just sensed Luke.
But if we go more in-depth into the films' representations of the Force, Vader only senses Luke when he's actively chasing him as a pilot; he comments that "The Force is strong with this one" only when Luke is using the Force to aim and fire his shot. For me, this implies that you can only sense other Force users powerfully or distinctly when they're actively using the Force. I know why fic prefers not to go that way - I for one have used the "Vader senses Luke immediately" plot to get Luke captured many a time - but it's a fun thing to consider. Also, the Force is a soft magic system. It does functionally whatever the story needs it to.* Which is why Vader can sense Luke approaching Endor in ROTJ when Palpatine couldn't, but he had to be told by an officer when Luke's ship was approaching Cloud City in ESB.** There are many examples where a Force user just didn't sense someone else. Luke not sensing Yoda in ESB, Obi-Wan not sensing Dooku's presence on Geonosis, the Jedi non sensing Maul in TPM, the Jedi not sensing Palpatine for the whole prequel trilogy.
You could argue that those examples are of trained Force wielders shielding themselves, but I would argue that Qui-Gon can't sense Anakin in TPM. He doesn't show interest in Anakin until after they've left Watto's shop and Anakin starts talking to him. Anakin is the literal son of the Force and the most powerful Force wielder ever, but Qui-Gon was talking to Watto for a while without batting an eyelash at the small supernova in his shop. He didn't start to suspect until he heard about his racing, his instincts, spoke to him to notice his insights. Then still he spoke to Shmi about him to confirm his suspicions that "he can see things before they happen", watched him while he flew, and took a midichlorian count. I think the Force might well have been nudging Qui-Gon to look at Anakin, to suspect something, but I don't think he sensed Anakin himself as Force sensitive - at least, not immediately. Which is how I think you can explain all the instances of people going "I felt his presence." They were either using the Force, or familiar as Force sensitives to the person sensing them, or the Force wielder in general had an instinct that there was something special about this person, I should pay attention...
This has been a long ramble, but the short answer is: I headcanon Leia as naturally good at shielding. I think it fills multiple Star Wars plot holes.***
But I think it's also worth interrogating the fact that fandom seems to approach and conceptualise of the Force as a hard magic system, with clearly defined rules, rather than the soft magic system it is.**** Anything goes in Star Wars! It can be annoying if the writing doesn't sell it well enough, but I really love that aspect of the worldbuilding. And considering that the Force is a big fat plot device as well as giving people magical instincts for things that are Plot Relevant and things that aren't, I think it's a lot more interesting to consider that the Force isn't a superpower that lets you sense everything. Vader didn't detect Leia simply because he didn't. Sometimes they fail to do that. And it allows you to show growth in character and situation when that fact changes. Vader doesn't sense Luke in ANH until he's Plot Relevant to Vader's personal story. He doesn't sense Luke until he's fighting him in ESB. But in ROTJ, once they're both invested in their relationship and fated to meet, they're drawn together like stars caught in a mutual orbit.
That's the explanation I prefer. Because although it's less consistent, it's not unbelievable. It leaves uncertainty, mysticism, the chance for exploration in the galaxy. And most importantly, it tells a damn good story. Which, while this may not be true of people who love collecting lore and figuring the galaxy out, is ultimately what I'm here for.
*This is why so many random new powers can get added and explored in later movies and such, and also why I don't really get het up about it when they do add them. It just depends how you incorporate that new power. Usually, if a villain suddenly has a new power no one knew about (like Palpatine's lightning in ROTJ) it just ups the stakes, while if a hero suddenly has a new power it can feel like it cheapens their victory, like they haven't earned it; a deus ex machina. So messing about with Force powers is fine, it just depends how you incorporate them in the story.
**Admittedly this can be explained by the bond being formed when Luke learned the truth, but you know what I mean.
***I was only talking about the movies here, but there's also that moment in the Kenobi series where she's captured by Inquisitors and still no one notices she's Force sensitive??? There's just a lot of moments like this littered all over Star Wars, so this headcanon covers a lot of them.
****I kept using the terms hard/soft magic system without really explaining it here, but here's some good videos to dig into it: Soft Magic Systems | Hard Magic Systems