so if he wasn't shown and Killer didn't even notice means two things
1: they aren't even capable of sensing such an obvious aura of not only Shanks but Ryokugyu as well
2: they were scared and didn't want to face these strong characters
either one makes Kid and Killer look like losers
1st: Killer was shown BEFORE Shanks unleashing his Haki, which means we don't know if he heard that or not. All we can say is that he didn't perceive Aramaki's, but did Aramaki even unleash a shit ton of haki? He was using his Devil Fruit.
We also see Killer sitting silently, while everybody around (Bepo, Shachi) is partying and pussies are going for him. To me it looks like he's on guard and patiently waiting.
Also, Killer's CoO being worst than Scabbards was already proven wrong 50 chapters ago, since Denjiro was the only one barely able to perceive them at last second and was shitting bricks. , while those were a joke for Killer both to perceive and to deflect.
So yeah, Killer's CoO overwhelms Denjiro's and is canon.
2nd: Kidd is not shown so you are simply speculating. He could appear next chapter and say that he noticed a familiar haki like Luffy did, but without that friendly connotation.
Also, you really believe that Kidd & Killer being scared is a viable option?

If you think so you misreaded every panel in which they were in this whole arc.
Should be clear at this point that they'd rather die trying than run or surrender.
You can exclude that they felt fear.
Option 1 ---> Kidd & Killer perceived Shanks haki and Kidd is about to impulsively try something, even tho its to late (Shanks ship is already sailing away)
Option 2 ---> Kidd & Killer perceived Shanks haki and mentally noted that he's their next target, but right now is to soon
Option 3 ---> Kidd & Killer didn't perceive Shanks haki, but it makes 0 sense