I have nothing against Sikh's, I've known a few for decades.
> There's no reason to think that Sikhs are explicitly going out of their way to hide something.
... other than a small, curved sword or dagger that initiated (Amritdhari) Sikhs are required to wear at all times as one of the Five Ks (articles of faith) ordained by Guru Gobind Singh in 1699 you mean?
It seems reasonable, and a number of actual Sikh's would actually agree, to guess that a Sikh who holds their faith to be a matter of importance would absolutely hide a symbolic dagger the size of a letter opener and capable of slitting a throat.
The question facing any border agent tasked with checking Sikh's, of course, is this specific Sikh one that will always try to carry a Kiran, or one that doesn't hold with all that articles of faith guff.
Honestly, I would just give them a pass to carry a ceremonial knife, if they could prove they were Sikhs and not someone pretending to be. But I guess that's why we can't have nice things and why the same rules have to apply to everyone. I think most reasonable people understand that they can't preserve every aspect of their personal beliefs or pride in a situation involving the safety of millions of people flying daily. Carrying a weapon is certainly a bit unusual as a pillar of faith, but there are plenty of others that could also be deemed antipathetic to the well functioning order of a modern society trying to move people safely from A to B. And the same way I would consider trained and licensed gun owners to be a relatively low threat and a rule-abiding group of citizens, that's how I would view Sikhs with their blades (or even more so). So if you're Amish, take a horse. If it's Shabbat, wait til Sunday. If you're the TSA and you want to be more efficient by discriminating, look at people who have no discrenable ideology, or those whose ideology actively conflicts with your mission of preventing attacks.